<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <roleRef roleURI="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLoss" xlink:href="biib-20151231.xsd#AccumulatedOtherComprehensiveIncomeLoss" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossDetails" xlink:href="biib-20151231.xsd#AccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossDetails1" xlink:href="biib-20151231.xsd#AccumulatedOtherComprehensiveIncomeLossDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossTables" xlink:href="biib-20151231.xsd#AccumulatedOtherComprehensiveIncomeLossTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/Acquisitions" xlink:href="biib-20151231.xsd#Acquisitions" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/AcquisitionsAcquisitionsDetails" xlink:href="biib-20151231.xsd#AcquisitionsAcquisitionsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/AcquisitionsAcquisitionsDetails1" xlink:href="biib-20151231.xsd#AcquisitionsAcquisitionsDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/AcquisitionsAcquisitionsTables" xlink:href="biib-20151231.xsd#AcquisitionsAcquisitionsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/AcquisitionsDetailsTextual" xlink:href="biib-20151231.xsd#AcquisitionsDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationships" xlink:href="biib-20151231.xsd#CollaborativeAndOtherRelationships" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsDetails3" xlink:href="biib-20151231.xsd#CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsDetails3" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsDetails" xlink:href="biib-20151231.xsd#CollaborativeAndOtherRelationshipsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsDetails1" xlink:href="biib-20151231.xsd#CollaborativeAndOtherRelationshipsDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsDetails2" xlink:href="biib-20151231.xsd#CollaborativeAndOtherRelationshipsDetails2" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsDetailsTextual" xlink:href="biib-20151231.xsd#CollaborativeAndOtherRelationshipsDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsTables" xlink:href="biib-20151231.xsd#CollaborativeAndOtherRelationshipsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CommitmentsAndContingencies" xlink:href="biib-20151231.xsd#CommitmentsAndContingencies" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CommitmentsAndContingenciesCommitmentsAndContingenciesDetails1" xlink:href="biib-20151231.xsd#CommitmentsAndContingenciesCommitmentsAndContingenciesDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CommitmentsAndContingenciesDetails" xlink:href="biib-20151231.xsd#CommitmentsAndContingenciesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CommitmentsAndContingenciesDetailsTextual" xlink:href="biib-20151231.xsd#CommitmentsAndContingenciesDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/CommitmentsAndContingenciesTables" xlink:href="biib-20151231.xsd#CommitmentsAndContingenciesTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ConsolidatedBalanceSheets" xlink:href="biib-20151231.xsd#ConsolidatedBalanceSheets" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:href="biib-20151231.xsd#ConsolidatedBalanceSheetsParenthetical" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsOfCashFlows" xlink:href="biib-20151231.xsd#ConsolidatedStatementsOfCashFlows" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsOfComprehensiveIncome" xlink:href="biib-20151231.xsd#ConsolidatedStatementsOfComprehensiveIncome" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsOfEquity" xlink:href="biib-20151231.xsd#ConsolidatedStatementsOfEquity" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ConsolidatedStatementsOfIncome" xlink:href="biib-20151231.xsd#ConsolidatedStatementsOfIncome" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/DerivativeInstruments" xlink:href="biib-20151231.xsd#DerivativeInstruments" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsDetails" xlink:href="biib-20151231.xsd#DerivativeInstrumentsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsDetails1" xlink:href="biib-20151231.xsd#DerivativeInstrumentsDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsDetailsTextual" xlink:href="biib-20151231.xsd#DerivativeInstrumentsDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/DerivativeInstrumentsTables" xlink:href="biib-20151231.xsd#DerivativeInstrumentsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/DocumentAndEntityInformation" xlink:href="biib-20151231.xsd#DocumentAndEntityInformation" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/EarningsPerShare" xlink:href="biib-20151231.xsd#EarningsPerShare" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/EarningsPerShareDetails" xlink:href="biib-20151231.xsd#EarningsPerShareDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/EarningsPerShareTables" xlink:href="biib-20151231.xsd#EarningsPerShareTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/EmployeeBenefitPlans" xlink:href="biib-20151231.xsd#EmployeeBenefitPlans" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/EmployeeBenefitPlansDetails" xlink:href="biib-20151231.xsd#EmployeeBenefitPlansDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/Equity" xlink:href="biib-20151231.xsd#Equity" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/EquityDetails" xlink:href="biib-20151231.xsd#EquityDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/EquityDetailsTextual" xlink:href="biib-20151231.xsd#EquityDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/EquityTables" xlink:href="biib-20151231.xsd#EquityTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurements" xlink:href="biib-20151231.xsd#FairValueMeasurements" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsDetails" xlink:href="biib-20151231.xsd#FairValueMeasurementsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsDetails1" xlink:href="biib-20151231.xsd#FairValueMeasurementsDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsDetails2" xlink:href="biib-20151231.xsd#FairValueMeasurementsDetails2" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsDetailsTextual" xlink:href="biib-20151231.xsd#FairValueMeasurementsDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FairValueMeasurementsTables" xlink:href="biib-20151231.xsd#FairValueMeasurementsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FinancialInstruments" xlink:href="biib-20151231.xsd#FinancialInstruments" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetails" xlink:href="biib-20151231.xsd#FinancialInstrumentsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetails1" xlink:href="biib-20151231.xsd#FinancialInstrumentsDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetails2" xlink:href="biib-20151231.xsd#FinancialInstrumentsDetails2" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetails3" xlink:href="biib-20151231.xsd#FinancialInstrumentsDetails3" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetailsTextual1" xlink:href="biib-20151231.xsd#FinancialInstrumentsDetailsTextual1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsDetailsTextual2" xlink:href="biib-20151231.xsd#FinancialInstrumentsDetailsTextual2" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/FinancialInstrumentsTables" xlink:href="biib-20151231.xsd#FinancialInstrumentsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/Guarantees" xlink:href="biib-20151231.xsd#Guarantees" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxes" xlink:href="biib-20151231.xsd#IncomeTaxes" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesDetails" xlink:href="biib-20151231.xsd#IncomeTaxesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesDetails1" xlink:href="biib-20151231.xsd#IncomeTaxesDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesDetails2" xlink:href="biib-20151231.xsd#IncomeTaxesDetails2" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesDetails3" xlink:href="biib-20151231.xsd#IncomeTaxesDetails3" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesDetailsTextual" xlink:href="biib-20151231.xsd#IncomeTaxesDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IncomeTaxesTables" xlink:href="biib-20151231.xsd#IncomeTaxesTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/Indebtedness" xlink:href="biib-20151231.xsd#Indebtedness" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IndebtednessDetails" xlink:href="biib-20151231.xsd#IndebtednessDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IndebtednessDetails1" xlink:href="biib-20151231.xsd#IndebtednessDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IndebtednessDetailsTextual" xlink:href="biib-20151231.xsd#IndebtednessDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IndebtednessTables" xlink:href="biib-20151231.xsd#IndebtednessTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwill" xlink:href="biib-20151231.xsd#IntangibleAssetsAndGoodwill" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails" xlink:href="biib-20151231.xsd#IntangibleAssetsAndGoodwillDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails1" xlink:href="biib-20151231.xsd#IntangibleAssetsAndGoodwillDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillTables" xlink:href="biib-20151231.xsd#IntangibleAssetsAndGoodwillTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/Inventory" xlink:href="biib-20151231.xsd#Inventory" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/InventoryDetails" xlink:href="biib-20151231.xsd#InventoryDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/InventoryDetailsTextual" xlink:href="biib-20151231.xsd#InventoryDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/InventoryTables" xlink:href="biib-20151231.xsd#InventoryTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/InvestmentsInVariableInterestEntities" xlink:href="biib-20151231.xsd#InvestmentsInVariableInterestEntities" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/InvestmentsInVariableInterestEntitiesDetails" xlink:href="biib-20151231.xsd#InvestmentsInVariableInterestEntitiesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/Litigation" xlink:href="biib-20151231.xsd#Litigation" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/LitigationDetails" xlink:href="biib-20151231.xsd#LitigationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail" xlink:href="biib-20151231.xsd#OtherConsolidatedFinancialStatementDetail" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailDetails" xlink:href="biib-20151231.xsd#OtherConsolidatedFinancialStatementDetailDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailDetailsTextual" xlink:href="biib-20151231.xsd#OtherConsolidatedFinancialStatementDetailDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables" xlink:href="biib-20151231.xsd#OtherConsolidatedFinancialStatementDetailTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/PropertyPlantAndEquipment" xlink:href="biib-20151231.xsd#PropertyPlantAndEquipment" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/PropertyPlantAndEquipmentDetails" xlink:href="biib-20151231.xsd#PropertyPlantAndEquipmentDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/PropertyPlantAndEquipmentDetailsTextual" xlink:href="biib-20151231.xsd#PropertyPlantAndEquipmentDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipmentDetails1" xlink:href="biib-20151231.xsd#PropertyPlantAndEquipmentPropertyPlantAndEquipmentDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/PropertyPlantAndEquipmentTables" xlink:href="biib-20151231.xsd#PropertyPlantAndEquipmentTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/QuarterlyFinancialDataUnaudited" xlink:href="biib-20151231.xsd#QuarterlyFinancialDataUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/QuarterlyFinancialDataUnauditedDetails" xlink:href="biib-20151231.xsd#QuarterlyFinancialDataUnauditedDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/QuarterlyFinancialDataUnauditedDetailsTextual" xlink:href="biib-20151231.xsd#QuarterlyFinancialDataUnauditedDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/QuarterlyFinancialDataUnauditedTables" xlink:href="biib-20151231.xsd#QuarterlyFinancialDataUnauditedTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/RestructuringRestructuring" xlink:href="biib-20151231.xsd#RestructuringRestructuring" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/RestructuringRestructuringDetails" xlink:href="biib-20151231.xsd#RestructuringRestructuringDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/RestructuringRestructuringDetailsTextual" xlink:href="biib-20151231.xsd#RestructuringRestructuringDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/RestructuringRestructuringTables" xlink:href="biib-20151231.xsd#RestructuringRestructuringTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/RevenueReserves" xlink:href="biib-20151231.xsd#RevenueReserves" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/RevenueReservesDetails" xlink:href="biib-20151231.xsd#RevenueReservesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/RevenueReservesDetails1" xlink:href="biib-20151231.xsd#RevenueReservesDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/RevenueReservesTables" xlink:href="biib-20151231.xsd#RevenueReservesTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SegmentInformation" xlink:href="biib-20151231.xsd#SegmentInformation" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SegmentInformationDetails" xlink:href="biib-20151231.xsd#SegmentInformationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SegmentInformationDetails1" xlink:href="biib-20151231.xsd#SegmentInformationDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SegmentInformationDetailsTextual" xlink:href="biib-20151231.xsd#SegmentInformationDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SegmentInformationTables" xlink:href="biib-20151231.xsd#SegmentInformationTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPayments" xlink:href="biib-20151231.xsd#ShareBasedPayments" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails1" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails1" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails2" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails2" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails3" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails3" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails4" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails4" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails5" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails5" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails6" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails6" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails8" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails8" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetails9" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetails9" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsDetailsTextual" xlink:href="biib-20151231.xsd#ShareBasedPaymentsDetailsTextual" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsShareBasedPaymentsDetails7" xlink:href="biib-20151231.xsd#ShareBasedPaymentsShareBasedPaymentsDetails7" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/ShareBasedPaymentsTables" xlink:href="biib-20151231.xsd#ShareBasedPaymentsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPolicies" xlink:href="biib-20151231.xsd#SummaryOfSignificantAccountingPolicies" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesDetails" xlink:href="biib-20151231.xsd#SummaryOfSignificantAccountingPoliciesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:href="biib-20151231.xsd#SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="simple" />
  <roleRef roleURI="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:href="biib-20151231.xsd#SummaryOfSignificantAccountingPoliciesTables" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLoss" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E9D425F48F453E7322C638A35175E404" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_F367701BAC9B5C4E1E7938A3517510B9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E9D425F48F453E7322C638A35175E404" xlink:to="loc_us-gaap_StatementTable_F367701BAC9B5C4E1E7938A3517510B9" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_35EF3E62B0FEFD161CAE38A351757A2E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_F367701BAC9B5C4E1E7938A3517510B9" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_35EF3E62B0FEFD161CAE38A351757A2E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_D08DD666831095457F6D38A3517533FB_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_35EF3E62B0FEFD161CAE38A351757A2E" xlink:to="loc_us-gaap_EquityComponentDomain_D08DD666831095457F6D38A3517533FB_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_D08DD666831095457F6D38A3517533FB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_35EF3E62B0FEFD161CAE38A351757A2E" xlink:to="loc_us-gaap_EquityComponentDomain_D08DD666831095457F6D38A3517533FB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_4E9EBCBBEA4C0F47856B38A35175DE26" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D08DD666831095457F6D38A3517533FB" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_4E9EBCBBEA4C0F47856B38A35175DE26" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember" xlink:label="loc_us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember_52B8760C6E948BB127DC38A351759E97" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D08DD666831095457F6D38A3517533FB" xlink:to="loc_us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember_52B8760C6E948BB127DC38A351759E97" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember" xlink:label="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_B32F9CA4C5D75A635EEC38A35175DB81" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D08DD666831095457F6D38A3517533FB" xlink:to="loc_us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember_B32F9CA4C5D75A635EEC38A35175DB81" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedTranslationAdjustmentMember" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_9990C88D179E86E82ECF38A351752411" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_D08DD666831095457F6D38A3517533FB" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_9990C88D179E86E82ECF38A351752411" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_9CDE9C9203C719405EC138A35175CF30" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E9D425F48F453E7322C638A35175E404" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_9CDE9C9203C719405EC138A35175CF30" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_1A6766F0BBAE99D698FA38A351764483" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E9D425F48F453E7322C638A35175E404" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_1A6766F0BBAE99D698FA38A351764483" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_7D1D700EF560D4EE171838A35176923C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E9D425F48F453E7322C638A35175E404" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_7D1D700EF560D4EE171838A35176923C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_9B3EE7F07332C2FD20DC38A35176A287" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E9D425F48F453E7322C638A35175E404" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_9B3EE7F07332C2FD20DC38A35176A287" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_277C06FF2DF5065E96A9A32AB8B8CD99" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_9BB201B91433480EF93DA32AB8B79A47" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_277C06FF2DF5065E96A9A32AB8B8CD99" xlink:to="loc_us-gaap_StatementTable_9BB201B91433480EF93DA32AB8B79A47" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_927D80F05A693AEB0221A32AB8B7EA06" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_9BB201B91433480EF93DA32AB8B79A47" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_927D80F05A693AEB0221A32AB8B7EA06" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_24A298292EEA86A62B4AA32AB8B760D6_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_927D80F05A693AEB0221A32AB8B7EA06" xlink:to="loc_us-gaap_EquityComponentDomain_24A298292EEA86A62B4AA32AB8B760D6_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_24A298292EEA86A62B4AA32AB8B760D6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_927D80F05A693AEB0221A32AB8B7EA06" xlink:to="loc_us-gaap_EquityComponentDomain_24A298292EEA86A62B4AA32AB8B760D6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_E7F3341A93C7485FD0C0A32AB8B8BA61" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_24A298292EEA86A62B4AA32AB8B760D6" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_E7F3341A93C7485FD0C0A32AB8B8BA61" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember" xlink:label="loc_us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember_717DB635D96594086B23A32AB8B84639" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_24A298292EEA86A62B4AA32AB8B760D6" xlink:to="loc_us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember_717DB635D96594086B23A32AB8B84639" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_4085589AEFE4C3D47979A32AB8B83CCA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_9BB201B91433480EF93DA32AB8B79A47" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_4085589AEFE4C3D47979A32AB8B83CCA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_D5DB919A12A1E3100A64A32AB8B8EBB5_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_4085589AEFE4C3D47979A32AB8B83CCA" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_D5DB919A12A1E3100A64A32AB8B8EBB5_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_D5DB919A12A1E3100A64A32AB8B8EBB5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_4085589AEFE4C3D47979A32AB8B83CCA" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_D5DB919A12A1E3100A64A32AB8B8EBB5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_870F6DCF66B42EFC83BDA32AB8B851AD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_D5DB919A12A1E3100A64A32AB8B8EBB5" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_870F6DCF66B42EFC83BDA32AB8B851AD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_1C915A4C86FE38E85B05A32AB8B84C0C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_277C06FF2DF5065E96A9A32AB8B8CD99" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_1C915A4C86FE38E85B05A32AB8B84C0C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_01BEA23D64A5F84CDDB9A32AB8B840AA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_277C06FF2DF5065E96A9A32AB8B8CD99" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_01BEA23D64A5F84CDDB9A32AB8B840AA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_B2003F153C8493AD1985A32AB8B85FAC" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_277C06FF2DF5065E96A9A32AB8B8CD99" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_B2003F153C8493AD1985A32AB8B85FAC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_38261C774CF9A59A46F9A32AB8B89003" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_277C06FF2DF5065E96A9A32AB8B8CD99" xlink:to="loc_us-gaap_NetIncomeLoss_38261C774CF9A59A46F9A32AB8B89003" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/AccumulatedOtherComprehensiveIncomeLossTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/Acquisitions" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/AcquisitionsAcquisitionsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_4B0D1102F508681F589493224D3C3E85" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4C69EB01C1EA5F6171A193224D3C60D8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_4B0D1102F508681F589493224D3C3E85" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4C69EB01C1EA5F6171A193224D3C60D8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_9761638C0CA7F8673A4993224D3CE278" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4C69EB01C1EA5F6171A193224D3C60D8" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_9761638C0CA7F8673A4993224D3CE278" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9A12111ADD3A9D02671793224D3C4C6B_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9761638C0CA7F8673A4993224D3CE278" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9A12111ADD3A9D02671793224D3C4C6B_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9A12111ADD3A9D02671793224D3C4C6B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9761638C0CA7F8673A4993224D3CE278" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9A12111ADD3A9D02671793224D3C4C6B" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ConvergencePharmaceuticalsMember" xlink:label="loc_biib_ConvergencePharmaceuticalsMember_584D5A2C868F0CD1228393224D3C5B79" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9A12111ADD3A9D02671793224D3C4C6B" xlink:to="loc_biib_ConvergencePharmaceuticalsMember_584D5A2C868F0CD1228393224D3C5B79" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_E01CD3F64172105EF56C93224D3C6D7B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_4B0D1102F508681F589493224D3C3E85" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_E01CD3F64172105EF56C93224D3C6D7B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_E6E156F93A07B676308793224D3CEBDE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_4B0D1102F508681F589493224D3C3E85" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_E6E156F93A07B676308793224D3CEBDE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_26E9A90E23F7C3CBD8BC93224D3C32E8" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_4B0D1102F508681F589493224D3C3E85" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_26E9A90E23F7C3CBD8BC93224D3C32E8" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/AcquisitionsAcquisitionsDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_05EC6EDFE30C861BF48293224D39A961" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_8A92E498CFC50F86900993224D3843DE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_05EC6EDFE30C861BF48293224D39A961" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_8A92E498CFC50F86900993224D3843DE" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_605C477E7E9F5A673FF293224D395267" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_8A92E498CFC50F86900993224D3843DE" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_605C477E7E9F5A673FF293224D395267" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_BDBBB50B85EEBBED801793224D3900CA_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_605C477E7E9F5A673FF293224D395267" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_BDBBB50B85EEBBED801793224D3900CA_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_BDBBB50B85EEBBED801793224D3900CA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_605C477E7E9F5A673FF293224D395267" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_BDBBB50B85EEBBED801793224D3900CA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ConvergencePharmaceuticalsMember" xlink:label="loc_biib_ConvergencePharmaceuticalsMember_31A9E723ECE4DF4F8DCE93224D399891" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_BDBBB50B85EEBBED801793224D3900CA" xlink:to="loc_biib_ConvergencePharmaceuticalsMember_31A9E723ECE4DF4F8DCE93224D399891" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_B8540C3E76381D3FAD5493224D39FBBC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_05EC6EDFE30C861BF48293224D39A961" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_B8540C3E76381D3FAD5493224D39FBBC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_89103FACF1D0D4B9C26293224D3994E7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_05EC6EDFE30C861BF48293224D39A961" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_89103FACF1D0D4B9C26293224D3994E7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_198DB2409DE54C383EF293224D39048B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_05EC6EDFE30C861BF48293224D39A961" xlink:to="loc_us-gaap_Goodwill_198DB2409DE54C383EF293224D39048B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent_F0EAA334FFC051ADE0DC93224D39758D" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_05EC6EDFE30C861BF48293224D39A961" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent_F0EAA334FFC051ADE0DC93224D39758D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentAssets" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentAssets_99C58519B10AC5892C5793224D39EA53" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_05EC6EDFE30C861BF48293224D39A961" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentAssets_99C58519B10AC5892C5793224D39EA53" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_7D38838342A7A24D505A93224D391B19" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_05EC6EDFE30C861BF48293224D39A961" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_7D38838342A7A24D505A93224D391B19" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/AcquisitionsAcquisitionsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/AcquisitionsDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_25626C3330FCFFA8EF0B65DB84EF7F09" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_25626C3330FCFFA8EF0B65DB84EF7F09" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_A43E2990F525B65B424E65DB84EF4D1A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_25626C3330FCFFA8EF0B65DB84EF7F09" xlink:to="loc_us-gaap_ProductOrServiceAxis_A43E2990F525B65B424E65DB84EF4D1A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_01B3E61721E4222A7A4C65DB84EF29D9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_A43E2990F525B65B424E65DB84EF4D1A" xlink:to="loc_us-gaap_ProductsAndServicesDomain_01B3E61721E4222A7A4C65DB84EF29D9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_01B3E61721E4222A7A4C65DB84EF29D9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_A43E2990F525B65B424E65DB84EF4D1A" xlink:to="loc_us-gaap_ProductsAndServicesDomain_01B3E61721E4222A7A4C65DB84EF29D9" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RaxatrigineMember" xlink:label="loc_biib_RaxatrigineMember_914C5EE36A52C7A9F7CB65DB84EF5DC9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_01B3E61721E4222A7A4C65DB84EF29D9" xlink:to="loc_biib_RaxatrigineMember_914C5EE36A52C7A9F7CB65DB84EF5DC9" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NeuropathicpainindicationsMember" xlink:label="loc_biib_NeuropathicpainindicationsMember_F45DC58CC17DFC3574FF65DB84EF4DCD" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_01B3E61721E4222A7A4C65DB84EF29D9" xlink:to="loc_biib_NeuropathicpainindicationsMember_F45DC58CC17DFC3574FF65DB84EF4DCD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_C50877112869C6B94EFC65DB84EF81BA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_25626C3330FCFFA8EF0B65DB84EF7F09" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_C50877112869C6B94EFC65DB84EF81BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D4A9A1AEC81489672A1765DB84EF19A1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_C50877112869C6B94EFC65DB84EF81BA" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D4A9A1AEC81489672A1765DB84EF19A1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D4A9A1AEC81489672A1765DB84EF19A1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_C50877112869C6B94EFC65DB84EF81BA" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D4A9A1AEC81489672A1765DB84EF19A1" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ConvergencePharmaceuticalsMember" xlink:label="loc_biib_ConvergencePharmaceuticalsMember_582A8FF708C55581F95865DB84EF3854" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D4A9A1AEC81489672A1765DB84EF19A1" xlink:to="loc_biib_ConvergencePharmaceuticalsMember_582A8FF708C55581F95865DB84EF3854" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TysabriProductMember" xlink:label="loc_biib_TysabriProductMember_E1851822BCC3BA1FFC2765DB84EF889F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_D4A9A1AEC81489672A1765DB84EF19A1" xlink:to="loc_biib_TysabriProductMember_E1851822BCC3BA1FFC2765DB84EF889F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_8A0481D00C4A318D2C7165DB84F0C04E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_8A0481D00C4A318D2C7165DB84F0C04E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately" xlink:label="loc_biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately_0986DFF09A228A13CCCB65DB84F08FCE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately_0986DFF09A228A13CCCB65DB84F08FCE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_A7F685AA14D7DAD78E7A65DB84F03FC0" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_A7F685AA14D7DAD78E7A65DB84F03FC0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_88E6E843AF55C281FAC465DB84F0C24E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1_88E6E843AF55C281FAC465DB84F0C24E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GoodwillPeriodIncreaseDecrease" xlink:label="loc_us-gaap_GoodwillPeriodIncreaseDecrease_9222F5B584599A22563D65DB84F0001C" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_GoodwillPeriodIncreaseDecrease_9222F5B584599A22563D65DB84F0001C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsDiscountRate" xlink:label="loc_us-gaap_FairValueInputsDiscountRate_35EFFC671EFCA996157865DB84F0BE00" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_FairValueInputsDiscountRate_35EFFC671EFCA996157865DB84F0BE00" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_FF9E8217E2B35FBBF3C365DB84F0088D" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_FF9E8217E2B35FBBF3C365DB84F0088D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_4F9D94BC92C21F1B02F065DB84F0DA89" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_4F9D94BC92C21F1B02F065DB84F0DA89" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireIntangibleAssets" xlink:label="loc_us-gaap_PaymentsToAcquireIntangibleAssets_7EDF24EA0B9F59FCB40E65DB84F02611" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_PaymentsToAcquireIntangibleAssets_7EDF24EA0B9F59FCB40E65DB84F02611" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueCurrent" xlink:label="loc_us-gaap_DeferredRevenueCurrent_C78F65BF68E23F5486C465DB84F0AAEC" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_DeferredRevenueCurrent_C78F65BF68E23F5486C465DB84F0AAEC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_B9030EBAEF2F879AA62065DB84F0B1BE" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_B9030EBAEF2F879AA62065DB84F0B1BE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FutureContingentPaymentForFirst12Months" xlink:label="loc_biib_FutureContingentPaymentForFirst12Months_2D28165EC5D48327DA2065DB84F0CC40" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_biib_FutureContingentPaymentForFirst12Months_2D28165EC5D48327DA2065DB84F0CC40" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FutureContingentPaymentForAnnualWorldwideNetSalesUpTo20Billion" xlink:label="loc_biib_FutureContingentPaymentForAnnualWorldwideNetSalesUpTo20Billion_1207F3F98AB356C20B4C65DB84F0F103" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_biib_FutureContingentPaymentForAnnualWorldwideNetSalesUpTo20Billion_1207F3F98AB356C20B4C65DB84F0F103" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FutureContingentPaymentThreshold" xlink:label="loc_biib_FutureContingentPaymentThreshold_EB879259F15D73B381E465DB84F0F05E" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_biib_FutureContingentPaymentThreshold_EB879259F15D73B381E465DB84F0F05E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FutureContingentPaymentForAnnualWorldwideNetSalesThatExceed20Billion" xlink:label="loc_biib_FutureContingentPaymentForAnnualWorldwideNetSalesThatExceed20Billion_EFAF0F346EBA5B8C8E9265DB84F0ABFE" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_67F598EDA763D9EA59EE65DB84EFFC00" xlink:to="loc_biib_FutureContingentPaymentForAnnualWorldwideNetSalesThatExceed20Billion_EFAF0F346EBA5B8C8E9265DB84F0ABFE" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationships" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsCollaborativeAndOtherRelationshipsDetails3" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_53FC471D714412EA7E8A38A3548A9B26" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_5FC91CF438B82F485A0F38A3548A6CD5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_53FC471D714412EA7E8A38A3548A9B26" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_5FC91CF438B82F485A0F38A3548A6CD5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_FFB27D0F59D4CED4C6FC38A3548AADBA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_5FC91CF438B82F485A0F38A3548A6CD5" xlink:to="loc_us-gaap_TypeOfArrangementAxis_FFB27D0F59D4CED4C6FC38A3548AADBA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2D19567CFFB86C539F9138A3548AD7B2_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_FFB27D0F59D4CED4C6FC38A3548AADBA" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2D19567CFFB86C539F9138A3548AD7B2_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2D19567CFFB86C539F9138A3548AD7B2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_FFB27D0F59D4CED4C6FC38A3548AADBA" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2D19567CFFB86C539F9138A3548AD7B2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EisaiMember" xlink:label="loc_biib_EisaiMember_C03D2A42EDD6CF4C9D5038A3548A08BE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_2D19567CFFB86C539F9138A3548AD7B2" xlink:to="loc_biib_EisaiMember_C03D2A42EDD6CF4C9D5038A3548A08BE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpenseIncurredByCollaboration" xlink:label="loc_biib_ExpenseIncurredByCollaboration_74516694506A14A50B1638A3548AED0F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_53FC471D714412EA7E8A38A3548A9B26" xlink:to="loc_biib_ExpenseIncurredByCollaboration_74516694506A14A50B1638A3548AED0F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpenseReflectedWithinStatementsOfIncome" xlink:label="loc_biib_ExpenseReflectedWithinStatementsOfIncome_09F9C30431555CF33A0538A3548ACB3B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_53FC471D714412EA7E8A38A3548A9B26" xlink:to="loc_biib_ExpenseReflectedWithinStatementsOfIncome_09F9C30431555CF33A0538A3548ACB3B" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_6D8C144561649DB2ECD838A351246275" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_9D19A7F2EE1D8D14503B38A35123599E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_6D8C144561649DB2ECD838A351246275" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_9D19A7F2EE1D8D14503B38A35123599E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_E26700D2162DB73CB57E38A35123EC02" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_9D19A7F2EE1D8D14503B38A35123599E" xlink:to="loc_us-gaap_ProductOrServiceAxis_E26700D2162DB73CB57E38A35123EC02" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_62664D6ADAD3E0E0ABAE38A35123B590_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_E26700D2162DB73CB57E38A35123EC02" xlink:to="loc_us-gaap_ProductsAndServicesDomain_62664D6ADAD3E0E0ABAE38A35123B590_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_62664D6ADAD3E0E0ABAE38A35123B590" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_E26700D2162DB73CB57E38A35123EC02" xlink:to="loc_us-gaap_ProductsAndServicesDomain_62664D6ADAD3E0E0ABAE38A35123B590" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_GAZYVAMember" xlink:label="loc_biib_GAZYVAMember_4B5AB1A8264A4795196038A351239DF5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_62664D6ADAD3E0E0ABAE38A35123B590" xlink:to="loc_biib_GAZYVAMember_4B5AB1A8264A4795196038A351239DF5" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RituxanMember" xlink:label="loc_biib_RituxanMember_AD753FB0C5B56224770138A35123CCDB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_62664D6ADAD3E0E0ABAE38A35123B590" xlink:to="loc_biib_RituxanMember_AD753FB0C5B56224770138A35123CCDB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_10CB0B1E14F88F18A83438A35123DA8F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_9D19A7F2EE1D8D14503B38A35123599E" xlink:to="loc_us-gaap_TypeOfArrangementAxis_10CB0B1E14F88F18A83438A35123DA8F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_A146D3511E62A4D85B6538A351232432_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_10CB0B1E14F88F18A83438A35123DA8F" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_A146D3511E62A4D85B6538A351232432_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_A146D3511E62A4D85B6538A351232432" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_10CB0B1E14F88F18A83438A35123DA8F" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_A146D3511E62A4D85B6538A351232432" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RocheGroupGenentechMember" xlink:label="loc_biib_RocheGroupGenentechMember_0455D84FED67B7FE19E538A351230557" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_A146D3511E62A4D85B6538A351232432" xlink:to="loc_biib_RocheGroupGenentechMember_0455D84FED67B7FE19E538A351230557" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_14EABB81BBBEB19A7E7038A351241832" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_9D19A7F2EE1D8D14503B38A35123599E" xlink:to="loc_us-gaap_StatementGeographicalAxis_14EABB81BBBEB19A7E7038A351241832" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_6CD0A72A268074DD74C438A35124E129_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_14EABB81BBBEB19A7E7038A351241832" xlink:to="loc_us-gaap_SegmentGeographicalDomain_6CD0A72A268074DD74C438A35124E129_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_6CD0A72A268074DD74C438A35124E129" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_14EABB81BBBEB19A7E7038A351241832" xlink:to="loc_us-gaap_SegmentGeographicalDomain_6CD0A72A268074DD74C438A35124E129" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_US" xlink:label="loc_country_US_86F3ADC7D5DA419BC79638A351241BF8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_6CD0A72A268074DD74C438A35124E129" xlink:to="loc_country_US_86F3ADC7D5DA419BC79638A351241BF8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignCountryMember" xlink:label="loc_us-gaap_ForeignCountryMember_CB1D8BCDC01573BF6C4538A351245DB5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_6CD0A72A268074DD74C438A35124E129" xlink:to="loc_us-gaap_ForeignCountryMember_CB1D8BCDC01573BF6C4538A351245DB5" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CoPromotionProfitSharingFormulaAbstract" xlink:label="loc_biib_CoPromotionProfitSharingFormulaAbstract_206A2574F36DEAD5BCA138A35124F55F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_6D8C144561649DB2ECD838A351246275" xlink:to="loc_biib_CoPromotionProfitSharingFormulaAbstract_206A2574F36DEAD5BCA138A35124F55F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionOne" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionOne_72EE9E256276BFF1F78038A3512435E4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CoPromotionProfitSharingFormulaAbstract_206A2574F36DEAD5BCA138A35124F55F" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionOne_72EE9E256276BFF1F78038A3512435E4" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoAbstract" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoAbstract_7F908F1E8E6DEE52357238A351243863" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CoPromotionProfitSharingFormulaAbstract_206A2574F36DEAD5BCA138A35124F55F" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoAbstract_7F908F1E8E6DEE52357238A351243863" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionOne" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionOne_A76C1526278DB1FF798538A35124809F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoAbstract_7F908F1E8E6DEE52357238A351243863" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionOne_A76C1526278DB1FF798538A35124809F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionTwo" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionTwo_519A267EE42F8895C1BC38A3512402A2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoAbstract_7F908F1E8E6DEE52357238A351243863" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionTwo_519A267EE42F8895C1BC38A3512402A2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionThree" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionThree_E9C77CA29E45AC9E23B338A35124FAED" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoAbstract_7F908F1E8E6DEE52357238A351243863" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionThree_E9C77CA29E45AC9E23B338A35124FAED" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RevenuesFromUnconsolidatedJointBusinessAbstract" xlink:label="loc_biib_RevenuesFromUnconsolidatedJointBusinessAbstract_2FB19097E477B7D598A538A351249A93" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_6D8C144561649DB2ECD838A351246275" xlink:to="loc_biib_RevenuesFromUnconsolidatedJointBusinessAbstract_2FB19097E477B7D598A538A351249A93" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ShareOfCoPromotionProfits" xlink:label="loc_biib_ShareOfCoPromotionProfits_4DE6DEB5D896A1DE17C638A351248B06" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RevenuesFromUnconsolidatedJointBusinessAbstract_2FB19097E477B7D598A538A351249A93" xlink:to="loc_biib_ShareOfCoPromotionProfits_4DE6DEB5D896A1DE17C638A351248B06" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_B0981BC5E19E2FE7BFA838A35124AA59" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RevenuesFromUnconsolidatedJointBusinessAbstract_2FB19097E477B7D598A538A351249A93" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_B0981BC5E19E2FE7BFA838A35124AA59" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_UnconsolidatedJointBusiness" xlink:label="loc_biib_UnconsolidatedJointBusiness_271FB402D617EEE48A4B38A3512419F5" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RevenuesFromUnconsolidatedJointBusinessAbstract_2FB19097E477B7D598A538A351249A93" xlink:to="loc_biib_UnconsolidatedJointBusiness_271FB402D617EEE48A4B38A3512419F5" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_9049014DA4092D3573CF38A356D66046" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_D7BDD709A11BAD29E44F38A356D5C0FC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_9049014DA4092D3573CF38A356D66046" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_D7BDD709A11BAD29E44F38A356D5C0FC" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="biib-20151231.xsd#biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis" xlink:label="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis_6670AF6F06670508869B38A356D52A32" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_D7BDD709A11BAD29E44F38A356D5C0FC" xlink:to="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis_6670AF6F06670508869B38A356D52A32" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain" xlink:label="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain_697FE10B46032A5F048D38A356D5C390_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis_6670AF6F06670508869B38A356D52A32" xlink:to="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain_697FE10B46032A5F048D38A356D5C390_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain" xlink:label="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain_697FE10B46032A5F048D38A356D5C390" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryAxis_6670AF6F06670508869B38A356D52A32" xlink:to="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain_697FE10B46032A5F048D38A356D5C390" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SobiRateToBiogenOnNetSalesInSobiTerritoryMember" xlink:label="loc_biib_SobiRateToBiogenOnNetSalesInSobiTerritoryMember_52C2DFB5E9A28A552DED38A356D5F208" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain_697FE10B46032A5F048D38A356D5C390" xlink:to="loc_biib_SobiRateToBiogenOnNetSalesInSobiTerritoryMember_52C2DFB5E9A28A552DED38A356D5F208" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BiogenRateToSobiOnNetSalesInBiogenNorthAmericaTerritoryMember" xlink:label="loc_biib_BiogenRateToSobiOnNetSalesInBiogenNorthAmericaTerritoryMember_2C7188067A5169ADEEA038A356D5894D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain_697FE10B46032A5F048D38A356D5C390" xlink:to="loc_biib_BiogenRateToSobiOnNetSalesInBiogenNorthAmericaTerritoryMember_2C7188067A5169ADEEA038A356D5894D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BiogenRateToSobiOnNetSalesInBiogenDirectTerritoryMember" xlink:label="loc_biib_BiogenRateToSobiOnNetSalesInBiogenDirectTerritoryMember_712C86F78E572F826D5338A356D6A5FF" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain_697FE10B46032A5F048D38A356D5C390" xlink:to="loc_biib_BiogenRateToSobiOnNetSalesInBiogenDirectTerritoryMember_712C86F78E572F826D5338A356D6A5FF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BiogenRateToSobiOnNetRevenueFromBiogenDistributorTerritoryMember" xlink:label="loc_biib_BiogenRateToSobiOnNetRevenueFromBiogenDistributorTerritoryMember_11FB6B83DBE4F62E20DF38A356D69B96" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RoyaltyAndNetRevenueShareRatesByTerritoryDomain_697FE10B46032A5F048D38A356D5C390" xlink:to="loc_biib_BiogenRateToSobiOnNetRevenueFromBiogenDistributorTerritoryMember_11FB6B83DBE4F62E20DF38A356D69B96" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_6665FAF73F4F26F9EAE838A356D62AAF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_D7BDD709A11BAD29E44F38A356D5C0FC" xlink:to="loc_us-gaap_StatementGeographicalAxis_6665FAF73F4F26F9EAE838A356D62AAF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_104DC06FFE042F1A6F7B38A356D63DAA_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_6665FAF73F4F26F9EAE838A356D62AAF" xlink:to="loc_us-gaap_SegmentGeographicalDomain_104DC06FFE042F1A6F7B38A356D63DAA_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_104DC06FFE042F1A6F7B38A356D63DAA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_6665FAF73F4F26F9EAE838A356D62AAF" xlink:to="loc_us-gaap_SegmentGeographicalDomain_104DC06FFE042F1A6F7B38A356D63DAA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SobiTerritoryMember" xlink:label="loc_biib_SobiTerritoryMember_E5973DF826CE8783512A38A356D6CB0F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_104DC06FFE042F1A6F7B38A356D63DAA" xlink:to="loc_biib_SobiTerritoryMember_E5973DF826CE8783512A38A356D6CB0F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_131869F99B4F0E33119E38A356D62707" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_D7BDD709A11BAD29E44F38A356D5C0FC" xlink:to="loc_us-gaap_RangeAxis_131869F99B4F0E33119E38A356D62707" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_1ED4A051FC1C3729297D38A356D66081_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_131869F99B4F0E33119E38A356D62707" xlink:to="loc_us-gaap_RangeMember_1ED4A051FC1C3729297D38A356D66081_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_1ED4A051FC1C3729297D38A356D66081" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_131869F99B4F0E33119E38A356D62707" xlink:to="loc_us-gaap_RangeMember_1ED4A051FC1C3729297D38A356D66081" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_6A4B51C7F202BD4CD99938A356D6EF75" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_1ED4A051FC1C3729297D38A356D66081" xlink:to="loc_us-gaap_MaximumMember_6A4B51C7F202BD4CD99938A356D6EF75" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_4C45755CB3F0AA55400C38A356D6CA5A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_1ED4A051FC1C3729297D38A356D66081" xlink:to="loc_us-gaap_MinimumMember_4C45755CB3F0AA55400C38A356D6CA5A" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ReimbursementUnderAmendedAgreementAmountsPayableAbstract" xlink:label="loc_biib_ReimbursementUnderAmendedAgreementAmountsPayableAbstract_7C67588696D31E7D1F9938A356D68E70" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_9049014DA4092D3573CF38A356D66046" xlink:to="loc_biib_ReimbursementUnderAmendedAgreementAmountsPayableAbstract_7C67588696D31E7D1F9938A356D68E70" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ReimbursementUnderAmendedAgreementMethod" xlink:label="loc_biib_ReimbursementUnderAmendedAgreementMethod_6CD61DAD54FED3715B6938A356D63F4E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ReimbursementUnderAmendedAgreementAmountsPayableAbstract_7C67588696D31E7D1F9938A356D68E70" xlink:to="loc_biib_ReimbursementUnderAmendedAgreementMethod_6CD61DAD54FED3715B6938A356D63F4E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RatePriorToFirstCommercialSale" xlink:label="loc_biib_RatePriorToFirstCommercialSale_BED538263F231440DA2838A356D69282" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ReimbursementUnderAmendedAgreementAmountsPayableAbstract_7C67588696D31E7D1F9938A356D68E70" xlink:to="loc_biib_RatePriorToFirstCommercialSale_BED538263F231440DA2838A356D69282" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BaseRateAfterFirstCommercialSale" xlink:label="loc_biib_BaseRateAfterFirstCommercialSale_3FCE01CF9E6E97B5A99D38A356D6B9FC" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ReimbursementUnderAmendedAgreementAmountsPayableAbstract_7C67588696D31E7D1F9938A356D68E70" xlink:to="loc_biib_BaseRateAfterFirstCommercialSale_3FCE01CF9E6E97B5A99D38A356D6B9FC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RateDuringReimbursementPeriod" xlink:label="loc_biib_RateDuringReimbursementPeriod_DFF6B8226D1DC8A5BC0838A356D67522" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ReimbursementUnderAmendedAgreementAmountsPayableAbstract_7C67588696D31E7D1F9938A356D68E70" xlink:to="loc_biib_RateDuringReimbursementPeriod_DFF6B8226D1DC8A5BC0838A356D67522" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsDetails2" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_CC56B109C48B1665FBC038A355A8607A" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_2F92622CF27BF1C4800938A355A84869" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_CC56B109C48B1665FBC038A355A8607A" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_2F92622CF27BF1C4800938A355A84869" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_8D8EE0D3A0B271C016D238A355A82CF0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_2F92622CF27BF1C4800938A355A84869" xlink:to="loc_us-gaap_TypeOfArrangementAxis_8D8EE0D3A0B271C016D238A355A82CF0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_E28E947ABD46F94A083B38A355A8D0CB_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_8D8EE0D3A0B271C016D238A355A82CF0" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_E28E947ABD46F94A083B38A355A8D0CB_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_E28E947ABD46F94A083B38A355A8D0CB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_8D8EE0D3A0B271C016D238A355A82CF0" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_E28E947ABD46F94A083B38A355A8D0CB" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AbbvieMember" xlink:label="loc_biib_AbbvieMember_E617B6CC3C877B4351AB38A355A81BD1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_E28E947ABD46F94A083B38A355A8D0CB" xlink:to="loc_biib_AbbvieMember_E617B6CC3C877B4351AB38A355A81BD1" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpenseIncurredByCollaboration" xlink:label="loc_biib_ExpenseIncurredByCollaboration_FE5E24C5E6FFF8154E4438A355A861D8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_CC56B109C48B1665FBC038A355A8607A" xlink:to="loc_biib_ExpenseIncurredByCollaboration_FE5E24C5E6FFF8154E4438A355A861D8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpenseReflectedWithinStatementsOfIncome" xlink:label="loc_biib_ExpenseReflectedWithinStatementsOfIncome_B78A52685CDE4A5814A338A355A865CA" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_CC56B109C48B1665FBC038A355A8607A" xlink:to="loc_biib_ExpenseReflectedWithinStatementsOfIncome_B78A52685CDE4A5814A338A355A865CA" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_E0912BB6DDFEB12549C693D5C96EB6F6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_us-gaap_StatementGeographicalAxis_E0912BB6DDFEB12549C693D5C96EB6F6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_AEECFD82D62299124D9793D5C96E5FA6_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_E0912BB6DDFEB12549C693D5C96EB6F6" xlink:to="loc_us-gaap_SegmentGeographicalDomain_AEECFD82D62299124D9793D5C96E5FA6_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_AEECFD82D62299124D9793D5C96E5FA6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_E0912BB6DDFEB12549C693D5C96EB6F6" xlink:to="loc_us-gaap_SegmentGeographicalDomain_AEECFD82D62299124D9793D5C96E5FA6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignCountryMember" xlink:label="loc_us-gaap_ForeignCountryMember_80AE7224E0EC5D14079E93D5C96E6C84" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_AEECFD82D62299124D9793D5C96E5FA6" xlink:to="loc_us-gaap_ForeignCountryMember_80AE7224E0EC5D14079E93D5C96E6C84" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_US" xlink:label="loc_country_US_02D07D2C9CEA6EF4DBD693D5C96E1867" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_AEECFD82D62299124D9793D5C96E5FA6" xlink:to="loc_country_US_02D07D2C9CEA6EF4DBD693D5C96E1867" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_C3349773AA0D57BEB8B993D5C96EC538" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_C3349773AA0D57BEB8B993D5C96EC538" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_FE8730FCE8FA2CC590FD93D5C96FB578_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_C3349773AA0D57BEB8B993D5C96EC538" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_FE8730FCE8FA2CC590FD93D5C96FB578_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_FE8730FCE8FA2CC590FD93D5C96FB578" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_C3349773AA0D57BEB8B993D5C96EC538" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_FE8730FCE8FA2CC590FD93D5C96FB578" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_E0FFB83D9FE6FB03368793D5C96F1696" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_FE8730FCE8FA2CC590FD93D5C96FB578" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_E0FFB83D9FE6FB03368793D5C96F1696" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis" xlink:label="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_353E77C6D02DCF8B951893D5C96FF734" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_353E77C6D02DCF8B951893D5C96FF734" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LossContingencyNatureDomain" xlink:label="loc_us-gaap_LossContingencyNatureDomain_46A8009752A5B984C18893D5C96FEE7D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_353E77C6D02DCF8B951893D5C96FF734" xlink:to="loc_us-gaap_LossContingencyNatureDomain_46A8009752A5B984C18893D5C96FEE7D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LossContingencyNatureDomain" xlink:label="loc_us-gaap_LossContingencyNatureDomain_46A8009752A5B984C18893D5C96FEE7D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_353E77C6D02DCF8B951893D5C96FF734" xlink:to="loc_us-gaap_LossContingencyNatureDomain_46A8009752A5B984C18893D5C96FEE7D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_HoechstMember" xlink:label="loc_biib_HoechstMember_9787FDBA9337F3760D3E93D5C96FA543" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_46A8009752A5B984C18893D5C96FEE7D" xlink:to="loc_biib_HoechstMember_9787FDBA9337F3760D3E93D5C96FA543" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AdditionalProductsAxis" xlink:label="loc_biib_AdditionalProductsAxis_E3851D105C85384660D593D5C96F98D6" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_biib_AdditionalProductsAxis_E3851D105C85384660D593D5C96F98D6" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AdditionalProductsDomain" xlink:label="loc_biib_AdditionalProductsDomain_06DD47C969E3A03571D493D5C96F6545_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_AdditionalProductsAxis_E3851D105C85384660D593D5C96F98D6" xlink:to="loc_biib_AdditionalProductsDomain_06DD47C969E3A03571D493D5C96F6545_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AdditionalProductsDomain" xlink:label="loc_biib_AdditionalProductsDomain_06DD47C969E3A03571D493D5C96F6545" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_AdditionalProductsAxis_E3851D105C85384660D593D5C96F98D6" xlink:to="loc_biib_AdditionalProductsDomain_06DD47C969E3A03571D493D5C96F6545" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NewAntiCd20Member" xlink:label="loc_biib_NewAntiCd20Member_6AC8E63DCEFFC5DC6C6393D5C96F77F1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_AdditionalProductsDomain_06DD47C969E3A03571D493D5C96F6545" xlink:to="loc_biib_NewAntiCd20Member_6AC8E63DCEFFC5DC6C6393D5C96F77F1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_72D9824CCE950F40F21093D5C96F399E" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_72D9824CCE950F40F21093D5C96F399E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityMethodInvesteeNameDomain" xlink:label="loc_us-gaap_EquityMethodInvesteeNameDomain_B0FA555C1F86CE910CA193D5C96FEAEE_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_72D9824CCE950F40F21093D5C96F399E" xlink:to="loc_us-gaap_EquityMethodInvesteeNameDomain_B0FA555C1F86CE910CA193D5C96FEAEE_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityMethodInvesteeNameDomain" xlink:label="loc_us-gaap_EquityMethodInvesteeNameDomain_B0FA555C1F86CE910CA193D5C96FEAEE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_72D9824CCE950F40F21093D5C96F399E" xlink:to="loc_us-gaap_EquityMethodInvesteeNameDomain_B0FA555C1F86CE910CA193D5C96FEAEE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SamsungBiosimilarAgreementMember" xlink:label="loc_biib_SamsungBiosimilarAgreementMember_1FF89E58852BA33785A993D5C96F87C1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityMethodInvesteeNameDomain_B0FA555C1F86CE910CA193D5C96FEAEE" xlink:to="loc_biib_SamsungBiosimilarAgreementMember_1FF89E58852BA33785A993D5C96F87C1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_C1CC42211F03E706987B93D5C96FD83E" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_us-gaap_ProductOrServiceAxis_C1CC42211F03E706987B93D5C96FD83E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_C1CC42211F03E706987B93D5C96FD83E" xlink:to="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_C1CC42211F03E706987B93D5C96FD83E" xlink:to="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RituxanMember" xlink:label="loc_biib_RituxanMember_5ABFC98D8F2F43AB5C1D93D5C96FA970" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_RituxanMember_5ABFC98D8F2F43AB5C1D93D5C96FA970" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_GAZYVAMember" xlink:label="loc_biib_GAZYVAMember_7148DE974A124A82D98893D5C96F3750" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_GAZYVAMember_7148DE974A124A82D98893D5C96F3750" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NewAntiCd20Member" xlink:label="loc_biib_NewAntiCd20Member_8EA4B6907A83B7811DF393D5C96F8D5C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_NewAntiCd20Member_8EA4B6907A83B7811DF393D5C96F8D5C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ThirdPartyAntiCdTwentyMember" xlink:label="loc_biib_ThirdPartyAntiCdTwentyMember_3114A1838DD1F322077B93D5C9701C93" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_ThirdPartyAntiCdTwentyMember_3114A1838DD1F322077B93D5C9701C93" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ELOCTATEMember" xlink:label="loc_biib_ELOCTATEMember_BABCFA279ECCB60FEE6993D5C9709290" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_ELOCTATEMember_BABCFA279ECCB60FEE6993D5C9709290" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ALPROLIXMember" xlink:label="loc_biib_ALPROLIXMember_34C73C20FB27CAFA6C3793D5C9702889" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_ALPROLIXMember_34C73C20FB27CAFA6C3793D5C9702889" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_OcrelizumabMember" xlink:label="loc_biib_OcrelizumabMember_8DC2C6B53A41932DDAEF93D5C97034B0" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_OcrelizumabMember_8DC2C6B53A41932DDAEF93D5C97034B0" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SOD1Member" xlink:label="loc_biib_SOD1Member_EB60A87A0FB3E1E02A2793DFA04EC495" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_SOD1Member_EB60A87A0FB3E1E02A2793DFA04EC495" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DMPKMember" xlink:label="loc_biib_DMPKMember_DF85E0ED1B849170CEEE93D5C9704FCA" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_DMPKMember_DF85E0ED1B849170CEEE93D5C9704FCA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SMNRxMember" xlink:label="loc_biib_SMNRxMember_7A78831878BA77DB329293D5C970ADF0" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_SMNRxMember_7A78831878BA77DB329293D5C970ADF0" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_LeadprogramsMember" xlink:label="loc_biib_LeadprogramsMember_753B73C00DCDB9A4950A93D5C970D598" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_LeadprogramsMember_753B73C00DCDB9A4950A93D5C970D598" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DiscoveryprogramsMember" xlink:label="loc_biib_DiscoveryprogramsMember_30C901FB7EE2418F69A693D5C97076E5" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_DiscoveryprogramsMember_30C901FB7EE2418F69A693D5C97076E5" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BiosimilarsMember" xlink:label="loc_biib_BiosimilarsMember_10197333C26473E38D5093D5C970D2A4" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_8EE24C88724AAB062E5593D5C96F1D99" xlink:to="loc_biib_BiosimilarsMember_10197333C26473E38D5093D5C970D2A4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_A0B789E254BBE740009A93D5C97095FA" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_us-gaap_TypeOfArrangementAxis_A0B789E254BBE740009A93D5C97095FA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_A0B789E254BBE740009A93D5C97095FA" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_A0B789E254BBE740009A93D5C97095FA" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RocheGroupGenentechMember" xlink:label="loc_biib_RocheGroupGenentechMember_1C2551141E3D2D79B80C93D5C97023A7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_RocheGroupGenentechMember_1C2551141E3D2D79B80C93D5C97023A7" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_GenentechMember" xlink:label="loc_biib_GenentechMember_3A76385BB3F92D60E76093D5C970B6CF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_GenentechMember_3A76385BB3F92D60E76093D5C970B6CF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ElanMember" xlink:label="loc_biib_ElanMember_F8C67C8905D242E26AE793D5C970525E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_ElanMember_F8C67C8905D242E26AE793D5C970525E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AcordaMember" xlink:label="loc_biib_AcordaMember_4E92B1A6BF7AF627891893D5C9700BB8" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_AcordaMember_4E92B1A6BF7AF627891893D5C9700BB8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SwedishOrphanBiovitrumMember" xlink:label="loc_biib_SwedishOrphanBiovitrumMember_E18976E6EAC193F1670B93D5C9705535" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_SwedishOrphanBiovitrumMember_E18976E6EAC193F1670B93D5C9705535" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AbbvieMember" xlink:label="loc_biib_AbbvieMember_7603D7708BE1A05A352693D5C9705C45" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_AbbvieMember_7603D7708BE1A05A352693D5C9705C45" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_IsisPharmaceuticalsMember" xlink:label="loc_biib_IsisPharmaceuticalsMember_F94324D1F70DE576704793D5C97041A4" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_IsisPharmaceuticalsMember_F94324D1F70DE576704793D5C97041A4" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EisaiMember" xlink:label="loc_biib_EisaiMember_21827AB91F12B494258D93D5C970E8A6" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_EisaiMember_21827AB91F12B494258D93D5C970E8A6" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SangamoBioSciencesMember" xlink:label="loc_biib_SangamoBioSciencesMember_B99A805243157ABB445893D5C9702A6C" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_SangamoBioSciencesMember_B99A805243157ABB445893D5C9702A6C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AGTCMember" xlink:label="loc_biib_AGTCMember_887AF927DE7A493B429793D5C971730D" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_AGTCMember_887AF927DE7A493B429793D5C971730D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MitsubishiTanabePharmaCorporationMember" xlink:label="loc_biib_MitsubishiTanabePharmaCorporationMember_FA9176EC03F833D8912793D5C97104E5" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_MitsubishiTanabePharmaCorporationMember_FA9176EC03F833D8912793D5C97104E5" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_OtherresearchanddiscoveryMember" xlink:label="loc_biib_OtherresearchanddiscoveryMember_3566344DF130A66F861E93D5C971A63B" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_OtherresearchanddiscoveryMember_3566344DF130A66F861E93D5C971A63B" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SamsungBiosimilarAgreementMember" xlink:label="loc_biib_SamsungBiosimilarAgreementMember_20F2750B592D1418D04293D5C9719874" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_3D2F342F8F0B489F4CD093D5C970CC01" xlink:to="loc_biib_SamsungBiosimilarAgreementMember_20F2750B592D1418D04293D5C9719874" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_B6420F58A851EB781E3A93D5C9715D0C" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_us-gaap_RangeAxis_B6420F58A851EB781E3A93D5C9715D0C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_8652B90BD740441D6AED93D5C971512E_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_B6420F58A851EB781E3A93D5C9715D0C" xlink:to="loc_us-gaap_RangeMember_8652B90BD740441D6AED93D5C971512E_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_8652B90BD740441D6AED93D5C971512E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_B6420F58A851EB781E3A93D5C9715D0C" xlink:to="loc_us-gaap_RangeMember_8652B90BD740441D6AED93D5C971512E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_656BB3D2295C575E68A193D5C971258A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_8652B90BD740441D6AED93D5C971512E" xlink:to="loc_us-gaap_MaximumMember_656BB3D2295C575E68A193D5C971258A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_AAA47C9143B57245747D93D5C9712C99" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_8652B90BD740441D6AED93D5C971512E" xlink:to="loc_us-gaap_MinimumMember_AAA47C9143B57245747D93D5C9712C99" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_22B6E6E4936353D6FF5293D5C9710618" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_1305351AE2995F0C0B1B93D5C96E7A2E" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_22B6E6E4936353D6FF5293D5C9710618" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FA3860EAF9C54F9DD19493D5C971DFEC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_22B6E6E4936353D6FF5293D5C9710618" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FA3860EAF9C54F9DD19493D5C971DFEC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FA3860EAF9C54F9DD19493D5C971DFEC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_22B6E6E4936353D6FF5293D5C9710618" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FA3860EAF9C54F9DD19493D5C971DFEC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BiogenIdecHemophiliaMember" xlink:label="loc_biib_BiogenIdecHemophiliaMember_C08125830F1C7595D31293D5C9712332" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FA3860EAF9C54F9DD19493D5C971DFEC" xlink:to="loc_biib_BiogenIdecHemophiliaMember_C08125830F1C7595D31293D5C9712332" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RoyaltiesTermsOfCollaborationAgreement" xlink:label="loc_biib_RoyaltiesTermsOfCollaborationAgreement_190F8FE5707A045C034493D5C971DC9F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_RoyaltiesTermsOfCollaborationAgreement_190F8FE5707A045C034493D5C971DC9F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CollaborationsTextualAbstract" xlink:label="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfRoyaltiesAsPerCollaboration" xlink:label="loc_biib_PercentageOfRoyaltiesAsPerCollaboration_5D0093F9D4DD7378419593D5C971DF0C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PercentageOfRoyaltiesAsPerCollaboration_5D0093F9D4DD7378419593D5C971DF0C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PeriodOfCollaborationAgreement" xlink:label="loc_biib_PeriodOfCollaborationAgreement_AEE58ED468B795DC264293D5C971CEE0" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PeriodOfCollaborationAgreement_AEE58ED468B795DC264293D5C971CEE0" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfDevelopmentAndCommercializationExpensesPayableRelated" xlink:label="loc_biib_PercentageOfDevelopmentAndCommercializationExpensesPayableRelated_795C0D84F80A6CD7D7F393D5C9728756" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PercentageOfDevelopmentAndCommercializationExpensesPayableRelated_795C0D84F80A6CD7D7F393D5C9728756" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FutureRoyaltiesPercentageToBeReceivedOnSaleOfProduct" xlink:label="loc_biib_FutureRoyaltiesPercentageToBeReceivedOnSaleOfProduct_F3F53D5CCF40D2AD5B8E93D5C972EB47" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_FutureRoyaltiesPercentageToBeReceivedOnSaleOfProduct_F3F53D5CCF40D2AD5B8E93D5C972EB47" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Reductioninroyaltyrate" xlink:label="loc_biib_Reductioninroyaltyrate_F00621A5F66C87870CA493D5C97288EC" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_Reductioninroyaltyrate_F00621A5F66C87870CA493D5C97288EC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsFirstFiftyMillion" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsFirstFiftyMillion_17C1BBA894D5D67B299C93D5C9720751" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsFirstFiftyMillion_17C1BBA894D5D67B299C93D5C9720751" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CoPromotionOperatingProfitThresholdToDetermineShareOfCoPromotionOperatingProfitPriorToAmendment" xlink:label="loc_biib_CoPromotionOperatingProfitThresholdToDetermineShareOfCoPromotionOperatingProfitPriorToAmendment_CB87872C91EF7EEFD73D93D5C9728092" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_CoPromotionOperatingProfitThresholdToDetermineShareOfCoPromotionOperatingProfitPriorToAmendment_CB87872C91EF7EEFD73D93D5C9728092" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ThresholdOfGrossSalesOfToBeAchievedInPrecedingTwelveConsecutiveMonthsUnderOptionOne" xlink:label="loc_biib_ThresholdOfGrossSalesOfToBeAchievedInPrecedingTwelveConsecutiveMonthsUnderOptionOne_7B25B94EF1114380917793D5C9727750" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ThresholdOfGrossSalesOfToBeAchievedInPrecedingTwelveConsecutiveMonthsUnderOptionOne_7B25B94EF1114380917793D5C9727750" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ThresholdOfGrossSalesToBeAchievedInAnyTwelveConsecutiveMonthsUnderOptionOne" xlink:label="loc_biib_ThresholdOfGrossSalesToBeAchievedInAnyTwelveConsecutiveMonthsUnderOptionOne_24C275EF6FD786E9805A93D5C972B925" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ThresholdOfGrossSalesToBeAchievedInAnyTwelveConsecutiveMonthsUnderOptionOne_24C275EF6FD786E9805A93D5C972B925" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SalesTriggerGrossSalesThreshold" xlink:label="loc_biib_SalesTriggerGrossSalesThreshold_396FA6F8703B8821032293D5C9726CE2" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_SalesTriggerGrossSalesThreshold_396FA6F8703B8821032293D5C9726CE2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Futurepercentageofcopromotionoperatingprofits" xlink:label="loc_biib_Futurepercentageofcopromotionoperatingprofits_4F8D1CC7249F45D2C2E093D5C97230D8" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_Futurepercentageofcopromotionoperatingprofits_4F8D1CC7249F45D2C2E093D5C97230D8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingLosses" xlink:label="loc_biib_PercentageOfCoPromotionOperatingLosses_CBE5308D32EBE93A422993D5C97284B2" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PercentageOfCoPromotionOperatingLosses_CBE5308D32EBE93A422993D5C97284B2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DecreaseInShareOfCoPromotionProfitsDueToEstimatedCompensationDamages" xlink:label="loc_biib_DecreaseInShareOfCoPromotionProfitsDueToEstimatedCompensationDamages_A1ADDE750FB90CB8FCEE93D5C972FA4E" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_DecreaseInShareOfCoPromotionProfitsDueToEstimatedCompensationDamages_A1ADDE750FB90CB8FCEE93D5C972FA4E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_B3C06B5FC540DDDAEAE893D5C972A9FB" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_B3C06B5FC540DDDAEAE893D5C972A9FB" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ShareOfCoPromotionProfits" xlink:label="loc_biib_ShareOfCoPromotionProfits_3029BCADED57FD2474BD93D5C9721F45" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ShareOfCoPromotionProfits_3029BCADED57FD2474BD93D5C9721F45" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillion" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillion_C9F22A32CB29E8F0B5E693D5C9720162" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillion_C9F22A32CB29E8F0B5E693D5C9720162" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_CD79E9D32061DEF323FE93D5C9725630" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_CD79E9D32061DEF323FE93D5C9725630" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ReimbursementsOfResearchAndDevelopmentsExpenseFromCollaborationPartner" xlink:label="loc_biib_ReimbursementsOfResearchAndDevelopmentsExpenseFromCollaborationPartner_97B8FE85B2AEDD3DAF9293D5C9720ADB" xlink:type="locator" />
    <definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ReimbursementsOfResearchAndDevelopmentsExpenseFromCollaborationPartner_97B8FE85B2AEDD3DAF9293D5C9720ADB" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ReimbursementsOfSellingGeneralAndAdministrativeExpensesFromCollaborationPartner" xlink:label="loc_biib_ReimbursementsOfSellingGeneralAndAdministrativeExpensesFromCollaborationPartner_FE328A3348C9C093511393D5C9720EAE" xlink:type="locator" />
    <definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ReimbursementsOfSellingGeneralAndAdministrativeExpensesFromCollaborationPartner_FE328A3348C9C093511393D5C9720EAE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AmountReflectedInCollaborationProfitSharingLineForCollaboration" xlink:label="loc_biib_AmountReflectedInCollaborationProfitSharingLineForCollaboration_FFBC347447608196092A93D5C972D40B" xlink:type="locator" />
    <definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_AmountReflectedInCollaborationProfitSharingLineForCollaboration_FFBC347447608196092A93D5C972D40B" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpectedAdditionalMilestonePaymentsOnSuccessfulAchievementOfRegulatoryAndCommercialSalesMilestones" xlink:label="loc_biib_ExpectedAdditionalMilestonePaymentsOnSuccessfulAchievementOfRegulatoryAndCommercialSalesMilestones_F48F09B0AFFAB4FA899293D5C972A015" xlink:type="locator" />
    <definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ExpectedAdditionalMilestonePaymentsOnSuccessfulAchievementOfRegulatoryAndCommercialSalesMilestones_F48F09B0AFFAB4FA899293D5C972A015" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpectedAdditionalMilestonePaymentsWhenMeetingCertainSalesThreshold" xlink:label="loc_biib_ExpectedAdditionalMilestonePaymentsWhenMeetingCertainSalesThreshold_4B9B23ABEA0867C9DA9893D5C9722ACA" xlink:type="locator" />
    <definitionArc order="22" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ExpectedAdditionalMilestonePaymentsWhenMeetingCertainSalesThreshold_4B9B23ABEA0867C9DA9893D5C9722ACA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ForeignSalesRequiredToTriggerMilestone" xlink:label="loc_biib_ForeignSalesRequiredToTriggerMilestone_198B74121E8F20EF985A93D5C97358CA" xlink:type="locator" />
    <definitionArc order="23" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ForeignSalesRequiredToTriggerMilestone_198B74121E8F20EF985A93D5C97358CA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpenseReflectedWithinStatementsOfIncome" xlink:label="loc_biib_ExpenseReflectedWithinStatementsOfIncome_5547D39B607D618C287793D5C97373E3" xlink:type="locator" />
    <definitionArc order="24" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ExpenseReflectedWithinStatementsOfIncome_5547D39B607D618C287793D5C97373E3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_9F98DD455CC00E73D81E93D5C973E83A" xlink:type="locator" />
    <definitionArc order="25" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_9F98DD455CC00E73D81E93D5C973E83A" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ConsiderationPerProductDeveloped" xlink:label="loc_biib_ConsiderationPerProductDeveloped_C8BE94CD385ADF5704AA93D5C973BB12" xlink:type="locator" />
    <definitionArc order="26" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ConsiderationPerProductDeveloped_C8BE94CD385ADF5704AA93D5C973BB12" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfReimbursementExpenses" xlink:label="loc_biib_PercentageOfReimbursementExpenses_E61E4789BF2BDEB75F5893D5C9736524" xlink:type="locator" />
    <definitionArc order="27" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PercentageOfReimbursementExpenses_E61E4789BF2BDEB75F5893D5C9736524" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfDevelopmentReimbursementExpenses" xlink:label="loc_biib_PercentageOfDevelopmentReimbursementExpenses_75E6598CDC58E09E045693D5C973C3FE" xlink:type="locator" />
    <definitionArc order="28" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PercentageOfDevelopmentReimbursementExpenses_75E6598CDC58E09E045693D5C973C3FE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ReimbursementCostAchievingPeriod" xlink:label="loc_biib_ReimbursementCostAchievingPeriod_AAB9D4EB8A2B433D4F9493D5C973CA35" xlink:type="locator" />
    <definitionArc order="29" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_ReimbursementCostAchievingPeriod_AAB9D4EB8A2B433D4F9493D5C973CA35" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TimePeriodForPayingRemainingBalanceDue" xlink:label="loc_biib_TimePeriodForPayingRemainingBalanceDue_3E75520ADE0C7484C0D293D5C9733DEF" xlink:type="locator" />
    <definitionArc order="30" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_TimePeriodForPayingRemainingBalanceDue_3E75520ADE0C7484C0D293D5C9733DEF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PeriodForGrantingOfApprovalByApprovingAuthority" xlink:label="loc_biib_PeriodForGrantingOfApprovalByApprovingAuthority_D71C2BE170A37B6EA15C93D5C97374F9" xlink:type="locator" />
    <definitionArc order="31" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_PeriodForGrantingOfApprovalByApprovingAuthority_D71C2BE170A37B6EA15C93D5C97374F9" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones" xlink:label="loc_biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones_7F4E7355A9FDD2E7AE0E93D5C9733222" xlink:type="locator" />
    <definitionArc order="32" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CollaborationsTextualAbstract_2F0FFAEDB2DCA5BD97B993D5C971B429" xlink:to="loc_biib_EstimatedAdditionalPaymentsUponAchievementOfDevelopmentAndCommercialMilestones_7F4E7355A9FDD2E7AE0E93D5C9733222" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Termofcollaborationagreement" xlink:label="loc_biib_Termofcollaborationagreement_7E0327C69FC6C80503DF93D5C9731F57" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_Termofcollaborationagreement_7E0327C69FC6C80503DF93D5C9731F57" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_UpfrontAndMilestonePaymentsMadeToCollaborativePartner" xlink:label="loc_biib_UpfrontAndMilestonePaymentsMadeToCollaborativePartner_84EFF9834D509D0D282793D5C97361D5" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_UpfrontAndMilestonePaymentsMadeToCollaborativePartner_84EFF9834D509D0D282793D5C97361D5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssets" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssets_411A5AE1D8A305ED569B93D5C9733210" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssets_411A5AE1D8A305ED569B93D5C9733210" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_LicenseFee" xlink:label="loc_biib_LicenseFee_DDEAF86502DABC80A2FB93D5C9739A03" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_LicenseFee_DDEAF86502DABC80A2FB93D5C9739A03" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpectedLicenseFeeAndRegulatoryMilestonePayments" xlink:label="loc_biib_ExpectedLicenseFeeAndRegulatoryMilestonePayments_9633E8F995FFE31AA99993D5C97379DF" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_ExpectedLicenseFeeAndRegulatoryMilestonePayments_9633E8F995FFE31AA99993D5C97379DF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Additionalmilestonepaymentsforproductcandidateusingadifferentmodality" xlink:label="loc_biib_Additionalmilestonepaymentsforproductcandidateusingadifferentmodality_00B7CFB4A6D8E7A5580593D5C973B44B" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_Additionalmilestonepaymentsforproductcandidateusingadifferentmodality_00B7CFB4A6D8E7A5580593D5C973B44B" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AdditionalMilestonePayment" xlink:label="loc_biib_AdditionalMilestonePayment_5359E696D056AAD6845793D5C9731824" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_AdditionalMilestonePayment_5359E696D056AAD6845793D5C9731824" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_LiabilityForFairValueOfOptionsGrantedUnderAgreement" xlink:label="loc_biib_LiabilityForFairValueOfOptionsGrantedUnderAgreement_9F898B3FFB2A3EAC7BD493D5C97397E4" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_LiabilityForFairValueOfOptionsGrantedUnderAgreement_9F898B3FFB2A3EAC7BD493D5C97397E4" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MilestonePaymentsMadeDuringPeriod" xlink:label="loc_biib_MilestonePaymentsMadeDuringPeriod_D2B92E233CAF715D8E3793D5C9742C8D" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_MilestonePaymentsMadeDuringPeriod_D2B92E233CAF715D8E3793D5C9742C8D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Purchaseofcommonstock" xlink:label="loc_biib_Purchaseofcommonstock_204914CB0D936E5B606693D5C974358A" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_Purchaseofcommonstock_204914CB0D936E5B606693D5C974358A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LicenseCosts" xlink:label="loc_us-gaap_LicenseCosts_D75559A20C8EF99E5F3393D5C9749A05" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_us-gaap_LicenseCosts_D75559A20C8EF99E5F3393D5C9749A05" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Researchanddevelopmentservices" xlink:label="loc_biib_Researchanddevelopmentservices_99B74768AB0842EC7AA893D5C9743B14" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_Researchanddevelopmentservices_99B74768AB0842EC7AA893D5C9743B14" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Premiumonequityinvestment" xlink:label="loc_biib_Premiumonequityinvestment_7AEA0BC77BF1919BC72293D5C974F0CC" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_Premiumonequityinvestment_7AEA0BC77BF1919BC72293D5C974F0CC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_InvestmentsByThirdPartyInJointVentureAsPerAgreement" xlink:label="loc_biib_InvestmentsByThirdPartyInJointVentureAsPerAgreement_E0F9354CB30B46E83DE193D5C974B15F" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_InvestmentsByThirdPartyInJointVentureAsPerAgreement_E0F9354CB30B46E83DE193D5C974B15F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_JointVentureOwnerShipPercentageByThirdParty" xlink:label="loc_biib_JointVentureOwnerShipPercentageByThirdParty_2239FEA998EFAF28783E93D5C9741605" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_JointVentureOwnerShipPercentageByThirdParty_2239FEA998EFAF28783E93D5C9741605" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityMethodInvestments" xlink:label="loc_us-gaap_EquityMethodInvestments_0C18E13973A6B1AD86FA93D5C974E26B" xlink:type="locator" />
    <definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_us-gaap_EquityMethodInvestments_0C18E13973A6B1AD86FA93D5C974E26B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_DC7C146A501F502454A893D5C97432DC" xlink:type="locator" />
    <definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_DC7C146A501F502454A893D5C97432DC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EquityMethodInvestmentOwnershipPercentageMaximum" xlink:label="loc_biib_EquityMethodInvestmentOwnershipPercentageMaximum_A88B55D3DDC16EA67F2D93D5C974E09E" xlink:type="locator" />
    <definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_EquityMethodInvestmentOwnershipPercentageMaximum_A88B55D3DDC16EA67F2D93D5C974E09E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_A282151B713A4322109E93D5C974EA68" xlink:type="locator" />
    <definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_A282151B713A4322109E93D5C974EA68" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EquityMethodInvestmentsExpectedProfitShare" xlink:label="loc_biib_EquityMethodInvestmentsExpectedProfitShare_796C264D3983462D515093D5C9741242" xlink:type="locator" />
    <definitionArc order="22" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_EquityMethodInvestmentsExpectedProfitShare_796C264D3983462D515093D5C9741242" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherSalesRevenueNet" xlink:label="loc_us-gaap_OtherSalesRevenueNet_C68E69952778AA89647293D5C974B18D" xlink:type="locator" />
    <definitionArc order="23" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_us-gaap_OtherSalesRevenueNet_C68E69952778AA89647293D5C974B18D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RoyaltyPeriodForSubstantiallyAllRemainingRoyaltyBearingSalesRestOfWorld" xlink:label="loc_biib_RoyaltyPeriodForSubstantiallyAllRemainingRoyaltyBearingSalesRestOfWorld_728B22946A7477DD245893D5C974B7D0" xlink:type="locator" />
    <definitionArc order="24" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_RoyaltyPeriodForSubstantiallyAllRemainingRoyaltyBearingSalesRestOfWorld_728B22946A7477DD245893D5C974B7D0" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionOne" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionOne_431E97FB34007EC5370993D5C974754E" xlink:type="locator" />
    <definitionArc order="25" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionTwoSubOptionOne_431E97FB34007EC5370993D5C974754E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionOne" xlink:label="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionOne_BC08D65BB20D6B1C7C0C93D5C9742013" xlink:type="locator" />
    <definitionArc order="26" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_02333A722653C772BFA993D5C9710F0D" xlink:to="loc_biib_PercentageOfCoPromotionOperatingProfitsGreaterThanFirstFiftyMillionOptionOne_BC08D65BB20D6B1C7C0C93D5C9742013" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/CollaborativeAndOtherRelationshipsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/CommitmentsAndContingencies" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/CommitmentsAndContingenciesCommitmentsAndContingenciesDetails1" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/CommitmentsAndContingenciesDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/CommitmentsAndContingenciesDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C62F48C8E07A30171AF393582DC6626F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C62F48C8E07A30171AF393582DC6626F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="biib-20151231.xsd#biib_CumulativeSalesLevelAxis" xlink:label="loc_biib_CumulativeSalesLevelAxis_2D27C0052C808E3564BF93582DC6DE0F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C62F48C8E07A30171AF393582DC6626F" xlink:to="loc_biib_CumulativeSalesLevelAxis_2D27C0052C808E3564BF93582DC6DE0F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CumulativeSalesLevelDomain" xlink:label="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_CumulativeSalesLevelAxis_2D27C0052C808E3564BF93582DC6DE0F" xlink:to="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CumulativeSalesLevelDomain" xlink:label="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_CumulativeSalesLevelAxis_2D27C0052C808E3564BF93582DC6DE0F" xlink:to="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SixbillionMember" xlink:label="loc_biib_SixbillionMember_525EE520AF11727B08BD93582DC65ED0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:to="loc_biib_SixbillionMember_525EE520AF11727B08BD93582DC65ED0" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FourbillionMember" xlink:label="loc_biib_FourbillionMember_9BB0B81D90F32132B1D193582DC64433" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:to="loc_biib_FourbillionMember_9BB0B81D90F32132B1D193582DC64433" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FivebillionMember" xlink:label="loc_biib_FivebillionMember_65191D8793EC1EC4B4BF93582DC6E83D" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:to="loc_biib_FivebillionMember_65191D8793EC1EC4B4BF93582DC6E83D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SevenbillionMember" xlink:label="loc_biib_SevenbillionMember_1170AA26EF39E5EEA2E393582DC6D036" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:to="loc_biib_SevenbillionMember_1170AA26EF39E5EEA2E393582DC6D036" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_OneBillionMember" xlink:label="loc_biib_OneBillionMember_566EFD185CD9CD9BBF9093582DC69A60" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:to="loc_biib_OneBillionMember_566EFD185CD9CD9BBF9093582DC69A60" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EachAdditionalOneBillionUpToTwentyBillionMember" xlink:label="loc_biib_EachAdditionalOneBillionUpToTwentyBillionMember_BAA88DF10D1FC6C4164E93582DC6F1C7" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:to="loc_biib_EachAdditionalOneBillionUpToTwentyBillionMember_BAA88DF10D1FC6C4164E93582DC6F1C7" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ThreeBillionMember" xlink:label="loc_biib_ThreeBillionMember_C4E5C0E49873C4DC9E4F93582DC6C38E" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CumulativeSalesLevelDomain_BE417A4D8A937975330293582DC66C46" xlink:to="loc_biib_ThreeBillionMember_C4E5C0E49873C4DC9E4F93582DC6C38E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_7596968419A326AEE05A93582DC633AF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C62F48C8E07A30171AF393582DC6626F" xlink:to="loc_us-gaap_ProductOrServiceAxis_7596968419A326AEE05A93582DC633AF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_AD967973D1B3FFB37ED593582DC6AFB7_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_7596968419A326AEE05A93582DC633AF" xlink:to="loc_us-gaap_ProductsAndServicesDomain_AD967973D1B3FFB37ED593582DC6AFB7_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_AD967973D1B3FFB37ED593582DC6AFB7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_7596968419A326AEE05A93582DC633AF" xlink:to="loc_us-gaap_ProductsAndServicesDomain_AD967973D1B3FFB37ED593582DC6AFB7" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TecfideraMember" xlink:label="loc_biib_TecfideraMember_F57DD1017638A9AFC55393582DC68F35" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_AD967973D1B3FFB37ED593582DC6AFB7" xlink:to="loc_biib_TecfideraMember_F57DD1017638A9AFC55393582DC68F35" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationAxis" xlink:label="loc_biib_FacilityLocationAxis_A7C5474E32473F2F37C493582DC6E73E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C62F48C8E07A30171AF393582DC6626F" xlink:to="loc_biib_FacilityLocationAxis_A7C5474E32473F2F37C493582DC6E73E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationDomain" xlink:label="loc_biib_FacilityLocationDomain_BC5EAA10C947B3F27B7393582DC63B37_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_FacilityLocationAxis_A7C5474E32473F2F37C493582DC6E73E" xlink:to="loc_biib_FacilityLocationDomain_BC5EAA10C947B3F27B7393582DC63B37_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationDomain" xlink:label="loc_biib_FacilityLocationDomain_BC5EAA10C947B3F27B7393582DC63B37" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_FacilityLocationAxis_A7C5474E32473F2F37C493582DC6E73E" xlink:to="loc_biib_FacilityLocationDomain_BC5EAA10C947B3F27B7393582DC63B37" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CambridgeLeasesMember" xlink:label="loc_biib_CambridgeLeasesMember_2E4B3E3607F704D2635A93582DC79948" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_FacilityLocationDomain_BC5EAA10C947B3F27B7393582DC63B37" xlink:to="loc_biib_CambridgeLeasesMember_2E4B3E3607F704D2635A93582DC79948" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SolothurnMember" xlink:label="loc_biib_SolothurnMember_300FE7AFB8AB7FE0F77793582DC70F1E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_FacilityLocationDomain_BC5EAA10C947B3F27B7393582DC63B37" xlink:to="loc_biib_SolothurnMember_300FE7AFB8AB7FE0F77793582DC70F1E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_3D4976770DC48C8E9C7493582DC7A8C3" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C62F48C8E07A30171AF393582DC6626F" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_3D4976770DC48C8E9C7493582DC7A8C3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_C0AB5D493CE2C1801CCC93582DC7F628_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_3D4976770DC48C8E9C7493582DC7A8C3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_C0AB5D493CE2C1801CCC93582DC7F628_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_C0AB5D493CE2C1801CCC93582DC7F628" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_3D4976770DC48C8E9C7493582DC7A8C3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_C0AB5D493CE2C1801CCC93582DC7F628" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TysabriProductMember" xlink:label="loc_biib_TysabriProductMember_CDD6F4F3C468F56F8C9493582DC758C6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_C0AB5D493CE2C1801CCC93582DC7F628" xlink:to="loc_biib_TysabriProductMember_CDD6F4F3C468F56F8C9493582DC758C6" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BiogenIdecHemophiliaMember" xlink:label="loc_biib_BiogenIdecHemophiliaMember_6AD528A5AAD4B1DDE81893582DC735FE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_C0AB5D493CE2C1801CCC93582DC7F628" xlink:to="loc_biib_BiogenIdecHemophiliaMember_6AD528A5AAD4B1DDE81893582DC735FE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FumapharmAgMember" xlink:label="loc_biib_FumapharmAgMember_092129E2A285AA2512BF93582DC73C7E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_C0AB5D493CE2C1801CCC93582DC7F628" xlink:to="loc_biib_FumapharmAgMember_092129E2A285AA2512BF93582DC73C7E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Chargesrecognizedtovacatebuilding" xlink:label="loc_biib_Chargesrecognizedtovacatebuilding_E099AAB2ADD0765AF52793582DC7F358" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:to="loc_biib_Chargesrecognizedtovacatebuilding_E099AAB2ADD0765AF52793582DC7F358" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitment" xlink:label="loc_us-gaap_OtherCommitment_42202F7006C2C3AA256093582DC7908F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:to="loc_us-gaap_OtherCommitment_42202F7006C2C3AA256093582DC7908F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_E7B5D6A3E1F1E8567E5993582DC7ACD2" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_E7B5D6A3E1F1E8567E5993582DC7ACD2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MilestonePaymentsMadeDuringPeriod" xlink:label="loc_biib_MilestonePaymentsMadeDuringPeriod_9112320EAD59CF89DF6393582DC79CD0" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:to="loc_biib_MilestonePaymentsMadeDuringPeriod_9112320EAD59CF89DF6393582DC79CD0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_1B4201E56846CE7A3E1393582DC762AF" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_1B4201E56846CE7A3E1393582DC762AF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_3C87651AC57DB8E9970E93582DC760FF" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_3C87651AC57DB8E9970E93582DC760FF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CumulativeSalesLevel" xlink:label="loc_biib_CumulativeSalesLevel_DECAA53444CC975EA80893582DC79AFD" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_1C46867B73668FB9751B93582DC72426" xlink:to="loc_biib_CumulativeSalesLevel_DECAA53444CC975EA80893582DC79AFD" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/CommitmentsAndContingenciesTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ConsolidatedBalanceSheets" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ConsolidatedStatementsOfCashFlows" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ConsolidatedStatementsOfComprehensiveIncome" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ConsolidatedStatementsOfEquity" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_B842F67491A086C87A04A34B6AEB890F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_StatementTable_B842F67491A086C87A04A34B6AEB890F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_7E353E4C27E42D2F33B9A34B6AEB3683" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_B842F67491A086C87A04A34B6AEB890F" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_7E353E4C27E42D2F33B9A34B6AEB3683" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_7E353E4C27E42D2F33B9A34B6AEB3683" xlink:to="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_7E353E4C27E42D2F33B9A34B6AEB3683" xlink:to="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ParentCompanyMember" xlink:label="loc_us-gaap_ParentCompanyMember_A7A7B90EFBE226DA8327A34B6AEB5B78" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:to="loc_us-gaap_ParentCompanyMember_A7A7B90EFBE226DA8327A34B6AEB5B78" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PreferredStockMember" xlink:label="loc_us-gaap_PreferredStockMember_5FF835D36D07DC1FF096A34B6AEB7B24" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:to="loc_us-gaap_PreferredStockMember_5FF835D36D07DC1FF096A34B6AEB7B24" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_C6B088FECEF4DDF46ACBA34B6AEB3F29" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:to="loc_us-gaap_CommonStockMember_C6B088FECEF4DDF46ACBA34B6AEB3F29" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_4F03A9D50C43526FD55CA34B6AEB881F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_4F03A9D50C43526FD55CA34B6AEB881F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_9DB159D9B7D49CB877C7A34B6AEBCDE5" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_9DB159D9B7D49CB877C7A34B6AEBCDE5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_BDE4C6F4CE32ECB82C74A34B6AEBF67D" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:to="loc_us-gaap_RetainedEarningsMember_BDE4C6F4CE32ECB82C74A34B6AEBF67D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockMember" xlink:label="loc_us-gaap_TreasuryStockMember_C24894674BFCFA3ED127A34B6AEB6162" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:to="loc_us-gaap_TreasuryStockMember_C24894674BFCFA3ED127A34B6AEB6162" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncontrollingInterestMember" xlink:label="loc_us-gaap_NoncontrollingInterestMember_EA06B0336302B1C3190FA34B6AEB4F94" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_DC163B6140C2F8BAEC3DA34B6AEB39F4" xlink:to="loc_us-gaap_NoncontrollingInterestMember_EA06B0336302B1C3190FA34B6AEB4F94" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_97BC60DE6EF5DD36733CA34B6AEB83B4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_97BC60DE6EF5DD36733CA34B6AEB83B4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharesIssued" xlink:label="loc_us-gaap_SharesIssued_B1A61E954B94C94DC3FCA34B6AEC3001" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_SharesIssued_B1A61E954B94C94DC3FCA34B6AEC3001" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProfitLoss" xlink:label="loc_us-gaap_ProfitLoss_183DB335637CF53E5443A34B6AECC9C2" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_ProfitLoss_183DB335637CF53E5443A34B6AECC9C2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_D0170E653F0CA6910A35A34B6AEC0ABE" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_D0170E653F0CA6910A35A34B6AEC0ABE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" xlink:label="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_276E7416052B33694D7BA34B6AEC456E" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders_276E7416052B33694D7BA34B6AEC456E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncontrollingInterestIncreaseFromBusinessCombination" xlink:label="loc_us-gaap_NoncontrollingInterestIncreaseFromBusinessCombination_C5A74CDEA1B97BF22C3DA34B6AEC35E1" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_NoncontrollingInterestIncreaseFromBusinessCombination_C5A74CDEA1B97BF22C3DA34B6AEC35E1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation" xlink:label="loc_us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation_89D028E32038AECE1805A34B6AEC6801" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation_89D028E32038AECE1805A34B6AEC6801" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_8641BFE0E2D04EA1E77BA34B6AECADA1" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_8641BFE0E2D04EA1E77BA34B6AECADA1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockSharesAcquired" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_0A9B30088B8475F83E89A34B6AECD0D7" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_0A9B30088B8475F83E89A34B6AECD0D7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockRetiredCostMethodAmount" xlink:label="loc_us-gaap_TreasuryStockRetiredCostMethodAmount_3DC274BAF9B69CB6ACDFA34B6AEC0028" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_TreasuryStockRetiredCostMethodAmount_3DC274BAF9B69CB6ACDFA34B6AEC0028" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_511A1E90666A9443AFE9A34B6AECCD8C" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_511A1E90666A9443AFE9A34B6AECCD8C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued_63DF82B7E187B57D09D2A34B6AECB6A5" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueTreasuryStockReissued_63DF82B7E187B57D09D2A34B6AECB6A5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued_444971711DD9519B8472A34B6AEC721A" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesTreasuryStockReissued_444971711DD9519B8472A34B6AEC721A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_DE49D532E3538C2C52C9A34B6AECA296" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_DE49D532E3538C2C52C9A34B6AECA296" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_9FDBD3B7B97FA1B6DF09A34B6AEC9B7B" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_9FDBD3B7B97FA1B6DF09A34B6AEC9B7B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_6027D8B5BBF741A18224A34B6AECACF7" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_6027D8B5BBF741A18224A34B6AECACF7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation_8F5B3AF00556D11CA4EEA34B6AEC3519" xlink:type="locator" />
    <definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_31E80DA6C29CDB423988A34B6AEBF915" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation_8F5B3AF00556D11CA4EEA34B6AEC3519" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ConsolidatedStatementsOfIncome" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/DerivativeInstruments" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/DerivativeInstrumentsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativesFairValueLineItems" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_157E95E22652ACDA14A493582EE0E2E3" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_8FD40407D73CE268ABD093582EDEA6EA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_157E95E22652ACDA14A493582EE0E2E3" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_8FD40407D73CE268ABD093582EDEA6EA" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CurrencyAxis" xlink:label="loc_us-gaap_CurrencyAxis_34E4B5A01D850AE75F9493582EDEC52D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_8FD40407D73CE268ABD093582EDEA6EA" xlink:to="loc_us-gaap_CurrencyAxis_34E4B5A01D850AE75F9493582EDEC52D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_AllCurrenciesDomain" xlink:label="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CurrencyAxis_34E4B5A01D850AE75F9493582EDEC52D" xlink:to="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_AllCurrenciesDomain" xlink:label="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CurrencyAxis_34E4B5A01D850AE75F9493582EDEC52D" xlink:to="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_EUR" xlink:label="loc_currency_EUR_612C5E3C929BEF4D334E93582EDEC642" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D" xlink:to="loc_currency_EUR_612C5E3C929BEF4D334E93582EDEC642" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_CHF" xlink:label="loc_currency_CHF_5EF1605A2DA90172BADB935EC0DFE4FF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D" xlink:to="loc_currency_CHF_5EF1605A2DA90172BADB935EC0DFE4FF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_CAD" xlink:label="loc_currency_CAD_78095A2F2D20A72CCD5793582EDF0AE0" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D" xlink:to="loc_currency_CAD_78095A2F2D20A72CCD5793582EDF0AE0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_GBP" xlink:label="loc_currency_GBP_E07E6C181EF3F8FADBD593582EDF7E4F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D" xlink:to="loc_currency_GBP_E07E6C181EF3F8FADBD593582EDF7E4F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_AUD" xlink:label="loc_currency_AUD_6C55CFF9B350EFAF89DD93582EDF343F" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D" xlink:to="loc_currency_AUD_6C55CFF9B350EFAF89DD93582EDF343F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_JPY" xlink:label="loc_currency_JPY_071D806A25A2A7139E0393582EDFEDAA" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_A79DF33D46A98A3728AD93582EDE675D" xlink:to="loc_currency_JPY_071D806A25A2A7139E0393582EDFEDAA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_B0B6A67A2896B380B69393582EDF815B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_8FD40407D73CE268ABD093582EDEA6EA" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_B0B6A67A2896B380B69393582EDF815B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_C9136B57B7C2A4FD16E493582EDFF4E7_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_B0B6A67A2896B380B69393582EDF815B" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_C9136B57B7C2A4FD16E493582EDFF4E7_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_C9136B57B7C2A4FD16E493582EDFF4E7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_B0B6A67A2896B380B69393582EDF815B" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_C9136B57B7C2A4FD16E493582EDFF4E7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignExchangeContractMember" xlink:label="loc_us-gaap_ForeignExchangeContractMember_899718E16A049BEF310893582EDFBBF5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_C9136B57B7C2A4FD16E493582EDFF4E7" xlink:to="loc_us-gaap_ForeignExchangeContractMember_899718E16A049BEF310893582EDFBBF5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_1200A372C26E388D187F93582EDF1456" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_8FD40407D73CE268ABD093582EDEA6EA" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_1200A372C26E388D187F93582EDF1456" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_7B907FA064D48EC8DA8093582EDFE5D3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1200A372C26E388D187F93582EDF1456" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_7B907FA064D48EC8DA8093582EDFE5D3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_7B907FA064D48EC8DA8093582EDFE5D3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_1200A372C26E388D187F93582EDF1456" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_7B907FA064D48EC8DA8093582EDFE5D3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCurrentAssetsMember" xlink:label="loc_us-gaap_OtherCurrentAssetsMember_B7C1AC47A03184B37ECD93582EDFCF64" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_7B907FA064D48EC8DA8093582EDFE5D3" xlink:to="loc_us-gaap_OtherCurrentAssetsMember_B7C1AC47A03184B37ECD93582EDFCF64" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherInvestmentsMember" xlink:label="loc_us-gaap_OtherInvestmentsMember_89342B8C8348CA322A2793582EDF6256" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_7B907FA064D48EC8DA8093582EDFE5D3" xlink:to="loc_us-gaap_OtherInvestmentsMember_89342B8C8348CA322A2793582EDF6256" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccruedLiabilitiesMember" xlink:label="loc_us-gaap_AccruedLiabilitiesMember_4F31AD62251905C135E593582EDFC242" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_7B907FA064D48EC8DA8093582EDFE5D3" xlink:to="loc_us-gaap_AccruedLiabilitiesMember_4F31AD62251905C135E593582EDFC242" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherNoncurrentLiabilitiesMember" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_5A4422CDF2BE8DD6C3CD93582EDF645D" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_7B907FA064D48EC8DA8093582EDFE5D3" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_5A4422CDF2BE8DD6C3CD93582EDF645D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="loc_us-gaap_HedgingDesignationAxis_FF04858B8D713FB2EDAE93582EDF45CA" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_8FD40407D73CE268ABD093582EDEA6EA" xlink:to="loc_us-gaap_HedgingDesignationAxis_FF04858B8D713FB2EDAE93582EDF45CA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_2CA42FBE5EB08EF0480A93582EDFA815_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_FF04858B8D713FB2EDAE93582EDF45CA" xlink:to="loc_us-gaap_HedgingDesignationDomain_2CA42FBE5EB08EF0480A93582EDFA815_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_2CA42FBE5EB08EF0480A93582EDFA815" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_FF04858B8D713FB2EDAE93582EDF45CA" xlink:to="loc_us-gaap_HedgingDesignationDomain_2CA42FBE5EB08EF0480A93582EDFA815" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_C06494185A7F5FCBC6D793582EDFF51C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_2CA42FBE5EB08EF0480A93582EDFA815" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_C06494185A7F5FCBC6D793582EDFF51C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="loc_us-gaap_NondesignatedMember_68564F20C6283362111193582EE0335F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_2CA42FBE5EB08EF0480A93582EDFA815" xlink:to="loc_us-gaap_NondesignatedMember_68564F20C6283362111193582EE0335F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd#invest_DerivativeNotionalAmount" xlink:label="loc_invest_DerivativeNotionalAmount_2493643E075823EA649A93582EE0A13B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_157E95E22652ACDA14A493582EE0E2E3" xlink:to="loc_invest_DerivativeNotionalAmount_2493643E075823EA649A93582EE0A13B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeAssetFairValueNetAbstract" xlink:label="loc_us-gaap_DerivativeAssetFairValueNetAbstract_BA0D55B3BD94999B81A893582EE0EAE5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_157E95E22652ACDA14A493582EE0E2E3" xlink:to="loc_us-gaap_DerivativeAssetFairValueNetAbstract_BA0D55B3BD94999B81A893582EE0EAE5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_E3C988842006D5EC77CF93582EE04BF1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetFairValueNetAbstract_BA0D55B3BD94999B81A893582EE0EAE5" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_E3C988842006D5EC77CF93582EE04BF1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_471E4D3FF4D07150885593582EE0ACCC" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeAssetFairValueNetAbstract_BA0D55B3BD94999B81A893582EE0EAE5" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_471E4D3FF4D07150885593582EE0ACCC" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/DerivativeInstrumentsDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossLineItems" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_573609C0E7606F68E9E293582ED84B77" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8BCD211E255AEAE9C10D93582ED7C6E1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_573609C0E7606F68E9E293582ED84B77" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8BCD211E255AEAE9C10D93582ED7C6E1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeByNatureAxis" xlink:label="loc_us-gaap_DerivativeByNatureAxis_23A0F49B692869B696EC93582ED75CFB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8BCD211E255AEAE9C10D93582ED7C6E1" xlink:to="loc_us-gaap_DerivativeByNatureAxis_23A0F49B692869B696EC93582ED75CFB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeNameDomain" xlink:label="loc_us-gaap_DerivativeNameDomain_C843415E3C92A6777ADC93582ED75625_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeByNatureAxis_23A0F49B692869B696EC93582ED75CFB" xlink:to="loc_us-gaap_DerivativeNameDomain_C843415E3C92A6777ADC93582ED75625_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeNameDomain" xlink:label="loc_us-gaap_DerivativeNameDomain_C843415E3C92A6777ADC93582ED75625" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeByNatureAxis_23A0F49B692869B696EC93582ED75CFB" xlink:to="loc_us-gaap_DerivativeNameDomain_C843415E3C92A6777ADC93582ED75625" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesMember" xlink:label="loc_us-gaap_SalesMember_D5FE4A484447B39E723993582ED8DBD5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeNameDomain_C843415E3C92A6777ADC93582ED75625" xlink:to="loc_us-gaap_SalesMember_D5FE4A484447B39E723993582ED8DBD5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_106AC38E187C97465AD493582ED80981" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8BCD211E255AEAE9C10D93582ED7C6E1" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_106AC38E187C97465AD493582ED80981" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_5B8DB93FCF69B13C7AB493582ED82CCC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_106AC38E187C97465AD493582ED80981" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_5B8DB93FCF69B13C7AB493582ED82CCC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_5B8DB93FCF69B13C7AB493582ED82CCC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_106AC38E187C97465AD493582ED80981" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_5B8DB93FCF69B13C7AB493582ED82CCC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignExchangeContractMember" xlink:label="loc_us-gaap_ForeignExchangeContractMember_67D4C0FC526B5368136793582ED8ECB1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_5B8DB93FCF69B13C7AB493582ED82CCC" xlink:to="loc_us-gaap_ForeignExchangeContractMember_67D4C0FC526B5368136793582ED8ECB1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_259B476FFD90E4F44B6393582ED8C989" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8BCD211E255AEAE9C10D93582ED7C6E1" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_259B476FFD90E4F44B6393582ED8C989" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_3095240D889CC37EF20393582ED863FB_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_259B476FFD90E4F44B6393582ED8C989" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_3095240D889CC37EF20393582ED863FB_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_3095240D889CC37EF20393582ED863FB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_259B476FFD90E4F44B6393582ED8C989" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_3095240D889CC37EF20393582ED863FB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesMember" xlink:label="loc_us-gaap_SalesMember_B9E94AB3F9E6CEE6F28B93582ED88A38" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_3095240D889CC37EF20393582ED863FB" xlink:to="loc_us-gaap_SalesMember_B9E94AB3F9E6CEE6F28B93582ED88A38" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpenseMember" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpenseMember_8F59E3D0669AD13C1DD093582ED8C574" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_3095240D889CC37EF20393582ED863FB" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpenseMember_8F59E3D0669AD13C1DD093582ED8C574" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNetAbstract" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNetAbstract_CA99F4236C15B27CE53D93582ED84AB4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_573609C0E7606F68E9E293582ED84B77" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNetAbstract_CA99F4236C15B27CE53D93582ED84AB4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet_CB23975FDAC09B0BA47593582ED80148" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNetAbstract_CA99F4236C15B27CE53D93582ED84AB4" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet_CB23975FDAC09B0BA47593582ED80148" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet_3038DD34BF4CB6913BD793582ED84CBF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNetAbstract_CA99F4236C15B27CE53D93582ED84AB4" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet_3038DD34BF4CB6913BD793582ED84CBF" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/DerivativeInstrumentsDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativesFairValueLineItems" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_4B0719102212A6FE38C193582ED3EAC4" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_B251B83F4A59DAA4B37293582ED2C099" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_4B0719102212A6FE38C193582ED3EAC4" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_B251B83F4A59DAA4B37293582ED2C099" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_87AA4E43DD1805A0DE4C93582ED2479D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_B251B83F4A59DAA4B37293582ED2C099" xlink:to="loc_us-gaap_DebtInstrumentAxis_87AA4E43DD1805A0DE4C93582ED2479D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7D4BC042658495E595CF93582ED2F9A8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_87AA4E43DD1805A0DE4C93582ED2479D" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7D4BC042658495E595CF93582ED2F9A8_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7D4BC042658495E595CF93582ED2F9A8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_87AA4E43DD1805A0DE4C93582ED2479D" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7D4BC042658495E595CF93582ED2F9A8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" xlink:label="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_7BBA6CD71E88B7C1192493582ED28147" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7D4BC042658495E595CF93582ED2F9A8" xlink:to="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_7BBA6CD71E88B7C1192493582ED28147" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" xlink:label="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_3B588C655EF07FBB87B293582ED26018" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7D4BC042658495E595CF93582ED2F9A8" xlink:to="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_3B588C655EF07FBB87B293582ED26018" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember" xlink:label="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_28A83B8F9AF16314F04593582ED29F43" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7D4BC042658495E595CF93582ED2F9A8" xlink:to="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_28A83B8F9AF16314F04593582ED29F43" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_595557D31D62610900DA93582ED2654A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_B251B83F4A59DAA4B37293582ED2C099" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_595557D31D62610900DA93582ED2654A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_B31B3EE9E3303C95075593582ED26427_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_595557D31D62610900DA93582ED2654A" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_B31B3EE9E3303C95075593582ED26427_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_B31B3EE9E3303C95075593582ED26427" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_595557D31D62610900DA93582ED2654A" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_B31B3EE9E3303C95075593582ED26427" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestRateContractMember" xlink:label="loc_us-gaap_InterestRateContractMember_D9B0A01FFF8BB388A7BB93582ED25D7F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_B31B3EE9E3303C95075593582ED26427" xlink:to="loc_us-gaap_InterestRateContractMember_D9B0A01FFF8BB388A7BB93582ED25D7F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestRateSwapMember" xlink:label="loc_us-gaap_InterestRateSwapMember_4236ED4A89FC2A66A3BB93582ED228F3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_B31B3EE9E3303C95075593582ED26427" xlink:to="loc_us-gaap_InterestRateSwapMember_4236ED4A89FC2A66A3BB93582ED228F3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="loc_us-gaap_HedgingDesignationAxis_401544A4EFD6E694D1D393582ED3867F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_B251B83F4A59DAA4B37293582ED2C099" xlink:to="loc_us-gaap_HedgingDesignationAxis_401544A4EFD6E694D1D393582ED3867F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_D753A0AFA9C471F95C3193582ED3AE95_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_401544A4EFD6E694D1D393582ED3867F" xlink:to="loc_us-gaap_HedgingDesignationDomain_D753A0AFA9C471F95C3193582ED3AE95_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="loc_us-gaap_HedgingDesignationDomain_D753A0AFA9C471F95C3193582ED3AE95" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_401544A4EFD6E694D1D393582ED3867F" xlink:to="loc_us-gaap_HedgingDesignationDomain_D753A0AFA9C471F95C3193582ED3AE95" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_3BBDC99CA2A4A196E85B93582ED3DECF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_D753A0AFA9C471F95C3193582ED3AE95" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_3BBDC99CA2A4A196E85B93582ED3DECF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="loc_us-gaap_NondesignatedMember_298685895F2BA95A8D5893582ED379C2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_D753A0AFA9C471F95C3193582ED3AE95" xlink:to="loc_us-gaap_NondesignatedMember_298685895F2BA95A8D5893582ED379C2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd#invest_DerivativeNotionalAmount" xlink:label="loc_invest_DerivativeNotionalAmount_C1FEDE170CF0343B4AB993582ED3969A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_4B0719102212A6FE38C193582ED3EAC4" xlink:to="loc_invest_DerivativeNotionalAmount_C1FEDE170CF0343B4AB993582ED3969A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_DC15059F0ED68DC4508E93582ED33FA3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_4B0719102212A6FE38C193582ED3EAC4" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_DC15059F0ED68DC4508E93582ED33FA3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_3426D062DABB67CA998D93582ED31D7A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_4B0719102212A6FE38C193582ED3EAC4" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_3426D062DABB67CA998D93582ED31D7A" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/DerivativeInstrumentsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/DocumentAndEntityInformation" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/EarningsPerShare" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/EarningsPerShareDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_05AE7F98B990A61A310038A3550A28F2" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4500351C8C6B01A0B0DB38A35509ABCD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_05AE7F98B990A61A310038A3550A28F2" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4500351C8C6B01A0B0DB38A35509ABCD" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_8D1435F0F78793BAD56838A3550926BA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4500351C8C6B01A0B0DB38A35509ABCD" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_8D1435F0F78793BAD56838A3550926BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_89E7E1DE0CD38BB57CDE38A3550AE632_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_8D1435F0F78793BAD56838A3550926BA" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_89E7E1DE0CD38BB57CDE38A3550AE632_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_89E7E1DE0CD38BB57CDE38A3550AE632" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_8D1435F0F78793BAD56838A3550926BA" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_89E7E1DE0CD38BB57CDE38A3550AE632" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_StockOptionsAndEmployeeStockPurchasePlanMember" xlink:label="loc_biib_StockOptionsAndEmployeeStockPurchasePlanMember_E596138FF9D1D90C6D6A38A3550A36AA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_89E7E1DE0CD38BB57CDE38A3550AE632" xlink:to="loc_biib_StockOptionsAndEmployeeStockPurchasePlanMember_E596138FF9D1D90C6D6A38A3550A36AA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TimeVestedRestrictedStockUnitsMember" xlink:label="loc_biib_TimeVestedRestrictedStockUnitsMember_63D2FFB0AC774D77F74938A3550A5AD8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_89E7E1DE0CD38BB57CDE38A3550AE632" xlink:to="loc_biib_TimeVestedRestrictedStockUnitsMember_63D2FFB0AC774D77F74938A3550A5AD8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MarketStockUnitsMember" xlink:label="loc_biib_MarketStockUnitsMember_B9E9C06845B8A899EDD738A3550A8F26" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_89E7E1DE0CD38BB57CDE38A3550AE632" xlink:to="loc_biib_MarketStockUnitsMember_B9E9C06845B8A899EDD738A3550A8F26" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BasicAndDilutedEarningsPerShareAbstract" xlink:label="loc_biib_BasicAndDilutedEarningsPerShareAbstract_83E3F9FD4944AE8A5F0B38A3550ACC14" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_05AE7F98B990A61A310038A3550A28F2" xlink:to="loc_biib_BasicAndDilutedEarningsPerShareAbstract_83E3F9FD4944AE8A5F0B38A3550ACC14" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NumeratorAbstract" xlink:label="loc_biib_NumeratorAbstract_30D2DD904D2C85047A8738A3550A4CA9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_BasicAndDilutedEarningsPerShareAbstract_83E3F9FD4944AE8A5F0B38A3550ACC14" xlink:to="loc_biib_NumeratorAbstract_30D2DD904D2C85047A8738A3550A4CA9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_E275DC2B1441440C473B38A3550A3BCE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_NumeratorAbstract_30D2DD904D2C85047A8738A3550A4CA9" xlink:to="loc_us-gaap_NetIncomeLoss_E275DC2B1441440C473B38A3550A3BCE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DenominatorAbstract" xlink:label="loc_biib_DenominatorAbstract_1F4BA4D7D4AAB2A7604738A3550A9883" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_BasicAndDilutedEarningsPerShareAbstract_83E3F9FD4944AE8A5F0B38A3550ACC14" xlink:to="loc_biib_DenominatorAbstract_1F4BA4D7D4AAB2A7604738A3550A9883" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_1D7ED7FBDA70998A596A38A3550A57E4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DenominatorAbstract_1F4BA4D7D4AAB2A7604738A3550A9883" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_1D7ED7FBDA70998A596A38A3550A57E4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DilutiveSecuritiesAbstract" xlink:label="loc_us-gaap_DilutiveSecuritiesAbstract_B133058C18B4E730717938A3550A2979" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DenominatorAbstract_1F4BA4D7D4AAB2A7604738A3550A9883" xlink:to="loc_us-gaap_DilutiveSecuritiesAbstract_B133058C18B4E730717938A3550A2979" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_5497B723E5308EBCC12138A3550A1A87" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DilutiveSecuritiesAbstract_B133058C18B4E730717938A3550A2979" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_5497B723E5308EBCC12138A3550A1A87" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_830CEFE18602F71ECCD738A3550ADD37" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DilutiveSecuritiesAbstract_B133058C18B4E730717938A3550A2979" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment_830CEFE18602F71ECCD738A3550ADD37" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_A2D107C5C7675DF3390238A3550AF32F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_DenominatorAbstract_1F4BA4D7D4AAB2A7604738A3550A9883" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_A2D107C5C7675DF3390238A3550AF32F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/EarningsPerShareTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/EmployeeBenefitPlans" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/EmployeeBenefitPlansDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_A16AB03A2A829C39D57F93582DBBCB7F" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_7EC1F275AD1483AD62AD93582DBAC85A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_A16AB03A2A829C39D57F93582DBBCB7F" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_7EC1F275AD1483AD62AD93582DBAC85A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis" xlink:label="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_1DEAD1AADAA6F1577D3193582DBAD285" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_7EC1F275AD1483AD62AD93582DBAC85A" xlink:to="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_1DEAD1AADAA6F1577D3193582DBAD285" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_4870CE80E049CFA57E8393582DBA03F6_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_1DEAD1AADAA6F1577D3193582DBAD285" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_4870CE80E049CFA57E8393582DBA03F6_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_4870CE80E049CFA57E8393582DBA03F6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_1DEAD1AADAA6F1577D3193582DBAD285" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_4870CE80E049CFA57E8393582DBA03F6" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SavingsPlanMember" xlink:label="loc_biib_SavingsPlanMember_10CEA71099B8F7DE07F593582DBAA4F3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_4870CE80E049CFA57E8393582DBA03F6" xlink:to="loc_biib_SavingsPlanMember_10CEA71099B8F7DE07F593582DBAA4F3" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DeferredCompensationPlanMember" xlink:label="loc_biib_DeferredCompensationPlanMember_1CCEB4031C08B6E21A7193582DBA9373" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_4870CE80E049CFA57E8393582DBA03F6" xlink:to="loc_biib_DeferredCompensationPlanMember_1CCEB4031C08B6E21A7193582DBA9373" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SwissplanMember" xlink:label="loc_biib_SwissplanMember_24CD85770BCA10B6692993582DBABFFC" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_4870CE80E049CFA57E8393582DBA03F6" xlink:to="loc_biib_SwissplanMember_24CD85770BCA10B6692993582DBABFFC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_GermanPlanMember" xlink:label="loc_biib_GermanPlanMember_DFED4B345FF92402935293582DBABF76" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_4870CE80E049CFA57E8393582DBA03F6" xlink:to="loc_biib_GermanPlanMember_DFED4B345FF92402935293582DBABF76" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MinimumQualifyingAgeForEmployeeBenefitPlan" xlink:label="loc_biib_MinimumQualifyingAgeForEmployeeBenefitPlan_5000C4E6886F72532E8593582DBB65A2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_A16AB03A2A829C39D57F93582DBBCB7F" xlink:to="loc_biib_MinimumQualifyingAgeForEmployeeBenefitPlan_5000C4E6886F72532E8593582DBB65A2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EmployeeBenefitPlansTextualAbstract" xlink:label="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_A16AB03A2A829C39D57F93582DBBCB7F" xlink:to="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedContributionPlanCostRecognized" xlink:label="loc_us-gaap_DefinedContributionPlanCostRecognized_C4FB52C1B1F1E463BE5A93582DBB1CBA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:to="loc_us-gaap_DefinedContributionPlanCostRecognized_C4FB52C1B1F1E463BE5A93582DBB1CBA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent" xlink:label="loc_us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent_287186BE44A245B4438B93582DBBF320" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:to="loc_us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent_287186BE44A245B4438B93582DBBF320" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfMinimumInvestmentReturn" xlink:label="loc_biib_PercentageOfMinimumInvestmentReturn_6DE96913D428B52E056393582DBBF643" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:to="loc_biib_PercentageOfMinimumInvestmentReturn_6DE96913D428B52E056393582DBBF643" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedBenefitPlanFundedStatusOfPlan" xlink:label="loc_us-gaap_DefinedBenefitPlanFundedStatusOfPlan_9710048810DB5A6A65CA93582DBB20E4" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:to="loc_us-gaap_DefinedBenefitPlanFundedStatusOfPlan_9710048810DB5A6A65CA93582DBB20E4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedBenefitPlanBenefitObligation" xlink:label="loc_us-gaap_DefinedBenefitPlanBenefitObligation_FACFAE28B6BFCC147B9493582DBBD6BC" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:to="loc_us-gaap_DefinedBenefitPlanBenefitObligation_FACFAE28B6BFCC147B9493582DBBD6BC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PensionExpense" xlink:label="loc_us-gaap_PensionExpense_56ACAC55FE31D81D312D93582DBB2D3B" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:to="loc_us-gaap_PensionExpense_56ACAC55FE31D81D312D93582DBB2D3B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_8CB8E5E71EA19683EDD693582DBB0AD2" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_EmployeeBenefitPlansTextualAbstract_7D0C0DD42B8582825D8393582DBBE1EC" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_8CB8E5E71EA19683EDD693582DBB0AD2" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/Equity" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/EquityDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/EquityDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_F84558E38180F726A7EA38A350DEB4A0" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_854F01725EA5CFD6635B38A350DDB069" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_F84558E38180F726A7EA38A350DEB4A0" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_854F01725EA5CFD6635B38A350DDB069" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_2843337B39C3D9FECCE238A350DE9A92" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_854F01725EA5CFD6635B38A350DDB069" xlink:to="loc_us-gaap_StatementClassOfStockAxis_2843337B39C3D9FECCE238A350DE9A92" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_DEFD71EB7ACBCE2603C538A350DE68DB_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_2843337B39C3D9FECCE238A350DE9A92" xlink:to="loc_us-gaap_ClassOfStockDomain_DEFD71EB7ACBCE2603C538A350DE68DB_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_DEFD71EB7ACBCE2603C538A350DE68DB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_2843337B39C3D9FECCE238A350DE9A92" xlink:to="loc_us-gaap_ClassOfStockDomain_DEFD71EB7ACBCE2603C538A350DE68DB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="loc_us-gaap_SeriesAPreferredStockMember_BD57FD73F04BCC7EB89338A350DE9BB3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_DEFD71EB7ACBCE2603C538A350DE68DB" xlink:to="loc_us-gaap_SeriesAPreferredStockMember_BD57FD73F04BCC7EB89338A350DE9BB3" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeriesXPreferredStockMember" xlink:label="loc_biib_SeriesXPreferredStockMember_9E44739AFBD5045646AD38A350DE9E9A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_DEFD71EB7ACBCE2603C538A350DE68DB" xlink:to="loc_biib_SeriesXPreferredStockMember_9E44739AFBD5045646AD38A350DE9E9A" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_UndesignatedPreferredStockMember" xlink:label="loc_biib_UndesignatedPreferredStockMember_8D0FB545910F61E0CEC138A350DE495E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_DEFD71EB7ACBCE2603C538A350DE68DB" xlink:to="loc_biib_UndesignatedPreferredStockMember_8D0FB545910F61E0CEC138A350DE495E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_9E3B50CEA26AE16D8BC538A350DE82FA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_F84558E38180F726A7EA38A350DEB4A0" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_9E3B50CEA26AE16D8BC538A350DE82FA" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/EquityTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurements" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CB0BBE2DA7A3AE99178C38A356B4AC68" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_57E469E45496C534E1E238A356B3D278" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CB0BBE2DA7A3AE99178C38A356B4AC68" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_57E469E45496C534E1E238A356B3D278" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_2A61573EE6301817F85C38A356B37F89" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_57E469E45496C534E1E238A356B3D278" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_2A61573EE6301817F85C38A356B37F89" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_54A9AE8BE9B847EC449A38A356B37002_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_2A61573EE6301817F85C38A356B37F89" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_54A9AE8BE9B847EC449A38A356B37002_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_54A9AE8BE9B847EC449A38A356B37002" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_2A61573EE6301817F85C38A356B37F89" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_54A9AE8BE9B847EC449A38A356B37002" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_912A5211016B15994A0938A356B34CDE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_54A9AE8BE9B847EC449A38A356B37002" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_912A5211016B15994A0938A356B34CDE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_BDFEAE4247B6DF7DD91238A356B39C45" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_54A9AE8BE9B847EC449A38A356B37002" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_BDFEAE4247B6DF7DD91238A356B39C45" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_83B9CD8CEE450A8E474738A356B3CF17" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_54A9AE8BE9B847EC449A38A356B37002" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_83B9CD8CEE450A8E474738A356B3CF17" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_A65656DC19C1972C9C6E38A356B34244" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_57E469E45496C534E1E238A356B3D278" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_A65656DC19C1972C9C6E38A356B34244" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_BBCE05DE2CB3CCB047C638A356B35715_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_A65656DC19C1972C9C6E38A356B34244" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_BBCE05DE2CB3CCB047C638A356B35715_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_BBCE05DE2CB3CCB047C638A356B35715" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_A65656DC19C1972C9C6E38A356B34244" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_BBCE05DE2CB3CCB047C638A356B35715" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_DEEABE05639E36828F0238A356B3C7E4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_BBCE05DE2CB3CCB047C638A356B35715" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_DEEABE05639E36828F0238A356B3C7E4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_2A58EF54765ACD13352738A356B3EBD9" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_57E469E45496C534E1E238A356B3D278" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_2A58EF54765ACD13352738A356B3EBD9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_5E70717DD42DCEF281BB38A356B3B66C_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_2A58EF54765ACD13352738A356B3EBD9" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_5E70717DD42DCEF281BB38A356B3B66C_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_5E70717DD42DCEF281BB38A356B3B66C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_2A58EF54765ACD13352738A356B3EBD9" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_5E70717DD42DCEF281BB38A356B3B66C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_BE4A1FACA42DC4D6EE1F38A356B37E0C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_5E70717DD42DCEF281BB38A356B3B66C" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_BE4A1FACA42DC4D6EE1F38A356B37E0C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_0955D638C7816BCB216138A356B324DD" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_5E70717DD42DCEF281BB38A356B3B66C" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_0955D638C7816BCB216138A356B324DD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AssetBackedSecuritiesMember" xlink:label="loc_us-gaap_AssetBackedSecuritiesMember_362E30C5F20DCB9149B138A356B31269" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_5E70717DD42DCEF281BB38A356B3B66C" xlink:to="loc_us-gaap_AssetBackedSecuritiesMember_362E30C5F20DCB9149B138A356B31269" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisAbstract" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisAbstract_785FFD586FB922C5947B38A356B43DC6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CB0BBE2DA7A3AE99178C38A356B4AC68" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisAbstract_785FFD586FB922C5947B38A356B43DC6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AssetsFairValueDisclosureAbstract" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_DE4D7BD92C6C798AC39F38A356B4AC02" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisAbstract_785FFD586FB922C5947B38A356B43DC6" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_DE4D7BD92C6C798AC39F38A356B4AC02" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_2435279020FE517F5D0138A356B4D467" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_DE4D7BD92C6C798AC39F38A356B4AC02" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_2435279020FE517F5D0138A356B4D467" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecurities_339DCBB7E8ECF5E073B938A356B40CFE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_DE4D7BD92C6C798AC39F38A356B4AC02" xlink:to="loc_us-gaap_AvailableForSaleSecurities_339DCBB7E8ECF5E073B938A356B40CFE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentsFairValueDisclosure" xlink:label="loc_us-gaap_InvestmentsFairValueDisclosure_29DB9608F328A6EBD47338A356B44497" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_DE4D7BD92C6C798AC39F38A356B4AC02" xlink:to="loc_us-gaap_InvestmentsFairValueDisclosure_29DB9608F328A6EBD47338A356B44497" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignCurrencyContractAssetFairValueDisclosure" xlink:label="loc_us-gaap_ForeignCurrencyContractAssetFairValueDisclosure_71DF90912EC9A7A3F03838A356B42724" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_DE4D7BD92C6C798AC39F38A356B4AC02" xlink:to="loc_us-gaap_ForeignCurrencyContractAssetFairValueDisclosure_71DF90912EC9A7A3F03838A356B42724" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PlanAssetsForDeferredCompensationFairValueDisclosure" xlink:label="loc_biib_PlanAssetsForDeferredCompensationFairValueDisclosure_D6613B9F2C7F5BDDD6E138A356B409F6" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_DE4D7BD92C6C798AC39F38A356B4AC02" xlink:to="loc_biib_PlanAssetsForDeferredCompensationFairValueDisclosure_D6613B9F2C7F5BDDD6E138A356B409F6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AssetsFairValueDisclosure" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_21703CC0597A5CED4AE638A356B4FA34" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_DE4D7BD92C6C798AC39F38A356B4AC02" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_21703CC0597A5CED4AE638A356B4FA34" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_D14D031534A103A6E7E638A356B40464" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisAbstract_785FFD586FB922C5947B38A356B43DC6" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_D14D031534A103A6E7E638A356B40464" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure" xlink:label="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_7F68FAB9F50E8E82BD0E38A356B41F5E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_D14D031534A103A6E7E638A356B40464" xlink:to="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_7F68FAB9F50E8E82BD0E38A356B41F5E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_FB8E4292A1916FDF3A7538A356B48513" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_D14D031534A103A6E7E638A356B40464" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_FB8E4292A1916FDF3A7538A356B48513" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LiabilitiesFairValueDisclosure" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_69ADC2C3331D96F2CC4238A356B46C0C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_D14D031534A103A6E7E638A356B40464" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_69ADC2C3331D96F2CC4238A356B46C0C" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_630C7362783DF34BE27A38A35498CE3C" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_6B0BBD99BA996D8DB29438A35497542E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_630C7362783DF34BE27A38A35498CE3C" xlink:to="loc_us-gaap_DebtInstrumentTable_6B0BBD99BA996D8DB29438A35497542E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_EFD5E184E353765869CC38A35497BE6C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_6B0BBD99BA996D8DB29438A35497542E" xlink:to="loc_us-gaap_DebtInstrumentAxis_EFD5E184E353765869CC38A35497BE6C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_EFD5E184E353765869CC38A35497BE6C" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_EFD5E184E353765869CC38A35497BE6C" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NotesPayableToFumedicaMember" xlink:label="loc_biib_NotesPayableToFumedicaMember_71EA1F8EA9FE10FDD1C038A3549791B8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912" xlink:to="loc_biib_NotesPayableToFumedicaMember_71EA1F8EA9FE10FDD1C038A3549791B8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember" xlink:label="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_C12B3F15C51547BDAA8738A3549866D0" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912" xlink:to="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_C12B3F15C51547BDAA8738A3549866D0" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember" xlink:label="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_AD25EC3E4CBB2845468038A354980258" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912" xlink:to="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_AD25EC3E4CBB2845468038A354980258" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember" xlink:label="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_C738D8D8E6C8932BFDE838A354987F33" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912" xlink:to="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_C738D8D8E6C8932BFDE838A354987F33" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" xlink:label="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_465F76B6DF0CCE4A281A38A354984590" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912" xlink:to="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_465F76B6DF0CCE4A281A38A354984590" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" xlink:label="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_E41D70EC4F87B067298238A3549879BF" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_517C6EF63E9ECD2D30A338A35497F912" xlink:to="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_E41D70EC4F87B067298238A3549879BF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SummaryOfFairAndCarryingValueOfDebtInstrumentsAbstract" xlink:label="loc_biib_SummaryOfFairAndCarryingValueOfDebtInstrumentsAbstract_14600032C3BE18CC2F1038A3549810F8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_630C7362783DF34BE27A38A35498CE3C" xlink:to="loc_biib_SummaryOfFairAndCarryingValueOfDebtInstrumentsAbstract_14600032C3BE18CC2F1038A3549810F8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NotesPayableFairValueDisclosure" xlink:label="loc_us-gaap_NotesPayableFairValueDisclosure_105FE6420C89276F94F038A3549888B0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfFairAndCarryingValueOfDebtInstrumentsAbstract_14600032C3BE18CC2F1038A3549810F8" xlink:to="loc_us-gaap_NotesPayableFairValueDisclosure_105FE6420C89276F94F038A3549888B0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentFairValue" xlink:label="loc_us-gaap_DebtInstrumentFairValue_9D00D83E8762E71ED67B38A354984735" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfFairAndCarryingValueOfDebtInstrumentsAbstract_14600032C3BE18CC2F1038A3549810F8" xlink:to="loc_us-gaap_DebtInstrumentFairValue_9D00D83E8762E71ED67B38A354984735" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsDetails2" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionContingentConsiderationLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_F8B4E65F98953E8080C638A353909564" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_3B80226A0DFCABA8904838A35390EFA6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_F8B4E65F98953E8080C638A353909564" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_3B80226A0DFCABA8904838A35390EFA6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_4CC7EB0562C3C980D6DF38A3539002C8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_3B80226A0DFCABA8904838A35390EFA6" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_4CC7EB0562C3C980D6DF38A3539002C8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_66B27713F238BFA3C9A938A3539081C5_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_4CC7EB0562C3C980D6DF38A3539002C8" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_66B27713F238BFA3C9A938A3539081C5_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_66B27713F238BFA3C9A938A3539081C5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_4CC7EB0562C3C980D6DF38A3539002C8" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_66B27713F238BFA3C9A938A3539081C5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_FA6549F79D91B65A085638A35390C55E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_66B27713F238BFA3C9A938A3539081C5" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_FA6549F79D91B65A085638A35390C55E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_1A2FCB21FDEE8350B30038A35390A4A1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_F8B4E65F98953E8080C638A353909564" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_1A2FCB21FDEE8350B30038A35390A4A1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_63998FE40E5274EEAC9A38A35390E1CE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_F8B4E65F98953E8080C638A353909564" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_63998FE40E5274EEAC9A38A35390E1CE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_BD0563565BF61C4A021438A35390F2A1" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_F8B4E65F98953E8080C638A353909564" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_BD0563565BF61C4A021438A35390F2A1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_8C89FC9EA4F9BD38911438A353904E5B" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_F8B4E65F98953E8080C638A353909564" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements_8C89FC9EA4F9BD38911438A353904E5B" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionContingentConsiderationLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_371F0C070B7EB1C4C87293D5CA9946FA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_371F0C070B7EB1C4C87293D5CA9946FA" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ContingentConsiderationByTypeAxis" xlink:label="loc_us-gaap_ContingentConsiderationByTypeAxis_011018B3FCBF4C97908B93D5CA9908E8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_371F0C070B7EB1C4C87293D5CA9946FA" xlink:to="loc_us-gaap_ContingentConsiderationByTypeAxis_011018B3FCBF4C97908B93D5CA9908E8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ContingentConsiderationTypeDomain" xlink:label="loc_us-gaap_ContingentConsiderationTypeDomain_B03F8C62B7525987D5D493D5CA991520_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContingentConsiderationByTypeAxis_011018B3FCBF4C97908B93D5CA9908E8" xlink:to="loc_us-gaap_ContingentConsiderationTypeDomain_B03F8C62B7525987D5D493D5CA991520_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ContingentConsiderationTypeDomain" xlink:label="loc_us-gaap_ContingentConsiderationTypeDomain_B03F8C62B7525987D5D493D5CA991520" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContingentConsiderationByTypeAxis_011018B3FCBF4C97908B93D5CA9908E8" xlink:to="loc_us-gaap_ContingentConsiderationTypeDomain_B03F8C62B7525987D5D493D5CA991520" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherNoncurrentLiabilitiesMember" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_0BDD28D1B7E85ADF522193D5CA99AAFB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContingentConsiderationTypeDomain_B03F8C62B7525987D5D493D5CA991520" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_0BDD28D1B7E85ADF522193D5CA99AAFB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccruedLiabilitiesMember" xlink:label="loc_us-gaap_AccruedLiabilitiesMember_CBCBC09117C58495F53C93D5CA9E8C09" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContingentConsiderationTypeDomain_B03F8C62B7525987D5D493D5CA991520" xlink:to="loc_us-gaap_AccruedLiabilitiesMember_CBCBC09117C58495F53C93D5CA9E8C09" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_0F0C2663004B22C4142193D5CA9E1234" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_371F0C070B7EB1C4C87293D5CA9946FA" xlink:to="loc_us-gaap_DebtInstrumentAxis_0F0C2663004B22C4142193D5CA9E1234" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_0F0C2663004B22C4142193D5CA9E1234" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_0F0C2663004B22C4142193D5CA9E1234" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember" xlink:label="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_891970B7B1B2F196C32893D5CA9EB28D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459" xlink:to="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_891970B7B1B2F196C32893D5CA9EB28D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember" xlink:label="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_A15A67A2E253C06460EE93D5CA9EB2F1" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459" xlink:to="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_A15A67A2E253C06460EE93D5CA9EB2F1" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember" xlink:label="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_E6357CE5393A7D4B39D093D5CA9EDE2A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459" xlink:to="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_E6357CE5393A7D4B39D093D5CA9EDE2A" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" xlink:label="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_44E05FF9C983FA23756F93D5CA9EB9FB" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459" xlink:to="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_44E05FF9C983FA23756F93D5CA9EB9FB" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" xlink:label="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_3928BDE4280506A0979193D5CA9E5BB3" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7ADBF1A4268D19C0129A93D5CA9E8459" xlink:to="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_3928BDE4280506A0979193D5CA9E5BB3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_BA3F4BEDB33E43386CE193D5CA9ED476" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionContingentConsiderationTable_371F0C070B7EB1C4C87293D5CA9946FA" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_BA3F4BEDB33E43386CE193D5CA9ED476" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F15C026E217607F45D5993D5CA9EC938_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_BA3F4BEDB33E43386CE193D5CA9ED476" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F15C026E217607F45D5993D5CA9EC938_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F15C026E217607F45D5993D5CA9EC938" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_BA3F4BEDB33E43386CE193D5CA9ED476" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F15C026E217607F45D5993D5CA9EC938" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ConvergencePharmaceuticalsMember" xlink:label="loc_biib_ConvergencePharmaceuticalsMember_2A5FB2D3C2E5B8F5459793D5CA9FA548" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F15C026E217607F45D5993D5CA9EC938" xlink:to="loc_biib_ConvergencePharmaceuticalsMember_2A5FB2D3C2E5B8F5459793D5CA9FA548" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_StromedixMember" xlink:label="loc_biib_StromedixMember_4695609B053A40CEF97393D5CA9FD1C2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F15C026E217607F45D5993D5CA9EC938" xlink:to="loc_biib_StromedixMember_4695609B053A40CEF97393D5CA9FD1C2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CorporateJointVentureInvestmentsMember" xlink:label="loc_biib_CorporateJointVentureInvestmentsMember_6888F403EE1C785E636B93D5CA9F477B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F15C026E217607F45D5993D5CA9EC938" xlink:to="loc_biib_CorporateJointVentureInvestmentsMember_6888F403EE1C785E636B93D5CA9F477B" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BiogenIdecInternationalNeuroscienceMember" xlink:label="loc_biib_BiogenIdecInternationalNeuroscienceMember_D68BD599A551A3C2E0AC93D5CA9FF945" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F15C026E217607F45D5993D5CA9EC938" xlink:to="loc_biib_BiogenIdecInternationalNeuroscienceMember_D68BD599A551A3C2E0AC93D5CA9FF945" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_43A07A945E07D0A6A79F93E6AD7F3521" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_43A07A945E07D0A6A79F93E6AD7F3521" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_AA3113422E0AEFCC3A5493D5CA9FECD2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiabilityNoncurrent_AA3113422E0AEFCC3A5493D5CA9FECD2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsChangesInValuationTechniques" xlink:label="loc_us-gaap_FairValueMeasurementsChangesInValuationTechniques_AC1C63097055A9DB3E2393D5CA9F7F39" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_FairValueMeasurementsChangesInValuationTechniques_AC1C63097055A9DB3E2393D5CA9F7F39" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_D5A8B9B9728864F5162D93D5CA9F51CB" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings_D5A8B9B9728864F5162D93D5CA9F51CB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_B817FAC06C684443C72293D5CA9FF8AD" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_B817FAC06C684443C72293D5CA9FF8AD" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately" xlink:label="loc_biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately_CCC28E552FB78D57D8F493D5CA9FCA52" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_biib_PotentialFutureMilestonePaymentsCommitmentToThirdPartyApproximately_CCC28E552FB78D57D8F493D5CA9FCA52" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_88BF551DE420FD3ACD0E93D5CA9F35CE" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_88BF551DE420FD3ACD0E93D5CA9F35CE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_5C23C7655E1A30EF6DF793D5CA9F3953" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_5C23C7655E1A30EF6DF793D5CA9F3953" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsDiscountRate" xlink:label="loc_us-gaap_FairValueInputsDiscountRate_1861DDB7767BB75CD97793D5CA9F02B8" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_FairValueInputsDiscountRate_1861DDB7767BB75CD97793D5CA9F02B8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MilestonePaymentsMadeDuringPeriod" xlink:label="loc_biib_MilestonePaymentsMadeDuringPeriod_E249EF1B31AEDBE4277593D5CA9FF1CA" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_biib_MilestonePaymentsMadeDuringPeriod_E249EF1B31AEDBE4277593D5CA9FF1CA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_EE8FAFA4F99938EC1F9893D5CA9FE032" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionContingentConsiderationLineItems_1FBD8BC1CDE875E6BFEA93D5CA9FE238" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_EE8FAFA4F99938EC1F9893D5CA9FE032" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/FairValueMeasurementsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstruments" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashAndCashEquivalentsLineItems" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_50DD7BB047E4D5B9B12693224C0B772D" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_36E61ABCE67E4925BCF293224C0627A6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_50DD7BB047E4D5B9B12693224C0B772D" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_36E61ABCE67E4925BCF293224C0627A6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentTypeAxis" xlink:label="loc_us-gaap_InvestmentTypeAxis_D1C96879004D120A41FE93224C06599E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_36E61ABCE67E4925BCF293224C0627A6" xlink:to="loc_us-gaap_InvestmentTypeAxis_D1C96879004D120A41FE93224C06599E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_79BF02278F6C17B128A093224C06A66B_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_D1C96879004D120A41FE93224C06599E" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_79BF02278F6C17B128A093224C06A66B_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_79BF02278F6C17B128A093224C06A66B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_D1C96879004D120A41FE93224C06599E" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_79BF02278F6C17B128A093224C06A66B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommercialPaperMember" xlink:label="loc_us-gaap_CommercialPaperMember_FB6ADD8CC68CBD351F4593224C0B7A70" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_79BF02278F6C17B128A093224C06A66B" xlink:to="loc_us-gaap_CommercialPaperMember_FB6ADD8CC68CBD351F4593224C0B7A70" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementsMember" xlink:label="loc_us-gaap_RepurchaseAgreementsMember_38931A31A77E907C2C0993224C0B2699" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_79BF02278F6C17B128A093224C06A66B" xlink:to="loc_us-gaap_RepurchaseAgreementsMember_38931A31A77E907C2C0993224C0B2699" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="loc_us-gaap_MoneyMarketFundsMember_AA07445EBD9ED8B52AA693224C0BD230" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_79BF02278F6C17B128A093224C06A66B" xlink:to="loc_us-gaap_MoneyMarketFundsMember_AA07445EBD9ED8B52AA693224C0BD230" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtSecuritiesMember" xlink:label="loc_us-gaap_DebtSecuritiesMember_6476AC24DF8547A4DD6693224C0BB051" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_79BF02278F6C17B128A093224C06A66B" xlink:to="loc_us-gaap_DebtSecuritiesMember_6476AC24DF8547A4DD6693224C0BB051" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashEquivalentsAtCarryingValueAbstract" xlink:label="loc_us-gaap_CashEquivalentsAtCarryingValueAbstract_63BF7DA00FA8964CBBB893224C0B33E0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_50DD7BB047E4D5B9B12693224C0B772D" xlink:to="loc_us-gaap_CashEquivalentsAtCarryingValueAbstract_63BF7DA00FA8964CBBB893224C0B33E0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashEquivalentsAtCarryingValue_EC9F04072C2FDE4678D193224C0B82E7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashEquivalentsAtCarryingValueAbstract_63BF7DA00FA8964CBBB893224C0B33E0" xlink:to="loc_us-gaap_CashEquivalentsAtCarryingValue_EC9F04072C2FDE4678D193224C0B82E7" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0C3587EC485D2B006E78A34B6A135499" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_EF589FF997A39165DD75A34B6A0E8844" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0C3587EC485D2B006E78A34B6A135499" xlink:to="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_EF589FF997A39165DD75A34B6A0E8844" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_C204162746A8575AAA92A34B6A0E945D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_EF589FF997A39165DD75A34B6A0E8844" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_C204162746A8575AAA92A34B6A0E945D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_C204162746A8575AAA92A34B6A0E945D" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_C204162746A8575AAA92A34B6A0E945D" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CorporateDebtSecuritiesCurrentMember" xlink:label="loc_biib_CorporateDebtSecuritiesCurrentMember_48035EC16EEEB30D3517A34B6A13F018" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:to="loc_biib_CorporateDebtSecuritiesCurrentMember_48035EC16EEEB30D3517A34B6A13F018" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CorporateDebtSecuritiesNonCurrentMember" xlink:label="loc_biib_CorporateDebtSecuritiesNonCurrentMember_2FF5B4F3834FFCFF8AA0A34B6A13A1CE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:to="loc_biib_CorporateDebtSecuritiesNonCurrentMember_2FF5B4F3834FFCFF8AA0A34B6A13A1CE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_GovernmentSecuritiesCurrentMember" xlink:label="loc_biib_GovernmentSecuritiesCurrentMember_73E250A0607C282E86CBA34B6A13AE74" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:to="loc_biib_GovernmentSecuritiesCurrentMember_73E250A0607C282E86CBA34B6A13AE74" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_GovernmentSecuritiesNonCurrentMember" xlink:label="loc_biib_GovernmentSecuritiesNonCurrentMember_377740B18487722700FEA34B6A138831" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:to="loc_biib_GovernmentSecuritiesNonCurrentMember_377740B18487722700FEA34B6A138831" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember" xlink:label="loc_biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember_E5DC0C2F9CBEE4A11A78A34B6A1384CD" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:to="loc_biib_MortgageAndOtherAssetBackedSecuritiesCurrentMember_E5DC0C2F9CBEE4A11A78A34B6A1384CD" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember" xlink:label="loc_biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember_F3A6F99A8C8A7626A270A34B6A13EEE9" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:to="loc_biib_MortgageAndOtherAssetBackedSecuritiesNonCurrentMember_F3A6F99A8C8A7626A270A34B6A13EEE9" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MarketableEquitySecuritiesMember" xlink:label="loc_biib_MarketableEquitySecuritiesMember_5940B366C07C79E18A29A34B6A1362C2" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_6D063C990EE30D370992A34B6A0E93E7" xlink:to="loc_biib_MarketableEquitySecuritiesMember_5940B366C07C79E18A29A34B6A1362C2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAbstract" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAbstract_EFA3EA49F348A33DDDE6A34B6A149E38" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0C3587EC485D2B006E78A34B6A135499" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAbstract_EFA3EA49F348A33DDDE6A34B6A149E38" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecurities_345C9693A6544936BD96A34B6A14597A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_EFA3EA49F348A33DDDE6A34B6A149E38" xlink:to="loc_us-gaap_AvailableForSaleSecurities_345C9693A6544936BD96A34B6A14597A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_06818972C6A31812A1C7A34B6A14BED5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_EFA3EA49F348A33DDDE6A34B6A149E38" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_06818972C6A31812A1C7A34B6A14BED5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_7C63AC6000C4B847B010A34B6A14D008" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_EFA3EA49F348A33DDDE6A34B6A149E38" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_7C63AC6000C4B847B010A34B6A14D008" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAmortizedCost" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAmortizedCost_B67596B5771210779D01A34B6A14B966" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_EFA3EA49F348A33DDDE6A34B6A149E38" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAmortizedCost_B67596B5771210779D01A34B6A14B966" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetails2" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetails3" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetailsTextual1" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsDetailsTextual2" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_BDE54CDFD747302D705F38A356F6A244" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01718A016490CF9071EF38A356F6DFC5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_BDE54CDFD747302D705F38A356F6A244" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01718A016490CF9071EF38A356F6DFC5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_6C47C226576F7437020938A356F6744F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01718A016490CF9071EF38A356F6DFC5" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_6C47C226576F7437020938A356F6744F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_047044B7E1D20A20F63038A356F650DA_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_6C47C226576F7437020938A356F6744F" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_047044B7E1D20A20F63038A356F650DA_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_047044B7E1D20A20F63038A356F650DA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_6C47C226576F7437020938A356F6744F" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_047044B7E1D20A20F63038A356F650DA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_StrategicInvestmentsMember" xlink:label="loc_biib_StrategicInvestmentsMember_AEB935940F11C0E90BF838A356F691D4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_047044B7E1D20A20F63038A356F650DA" xlink:to="loc_biib_StrategicInvestmentsMember_AEB935940F11C0E90BF838A356F691D4" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_StrategicInvestmentPortfolio" xlink:label="loc_biib_StrategicInvestmentPortfolio_10B6954D7E10E83B4BE538A356F6B8A1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_BDE54CDFD747302D705F38A356F6A244" xlink:to="loc_biib_StrategicInvestmentPortfolio_10B6954D7E10E83B4BE538A356F6B8A1" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/FinancialInstrumentsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/Guarantees" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxes" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesDetails1" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesDetails2" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesDetails3" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardLineItems" xlink:label="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardTable" xlink:label="loc_us-gaap_TaxCreditCarryforwardTable_B2C2EAED4AE128D95DD093582E2E2219" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_us-gaap_TaxCreditCarryforwardTable_B2C2EAED4AE128D95DD093582E2E2219" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_30208D169A56F678697293582E2E43A1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_TaxCreditCarryforwardTable_B2C2EAED4AE128D95DD093582E2E2219" xlink:to="loc_us-gaap_ProductOrServiceAxis_30208D169A56F678697293582E2E43A1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_3CF9F316829008BD64D393582E2EAB17_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_30208D169A56F678697293582E2E43A1" xlink:to="loc_us-gaap_ProductsAndServicesDomain_3CF9F316829008BD64D393582E2EAB17_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_3CF9F316829008BD64D393582E2EAB17" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_30208D169A56F678697293582E2E43A1" xlink:to="loc_us-gaap_ProductsAndServicesDomain_3CF9F316829008BD64D393582E2EAB17" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ZINBRYTAMember" xlink:label="loc_biib_ZINBRYTAMember_FE71A339EB72CED89E6493582E2FC7D1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_3CF9F316829008BD64D393582E2EAB17" xlink:to="loc_biib_ZINBRYTAMember_FE71A339EB72CED89E6493582E2FC7D1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardAxis" xlink:label="loc_us-gaap_TaxCreditCarryforwardAxis_A5B08BDDEE98876A464C93582E2F90CF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_TaxCreditCarryforwardTable_B2C2EAED4AE128D95DD093582E2E2219" xlink:to="loc_us-gaap_TaxCreditCarryforwardAxis_A5B08BDDEE98876A464C93582E2F90CF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_575ED5D5599A1FCF6F9793582E2FFE6B_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_A5B08BDDEE98876A464C93582E2F90CF" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_575ED5D5599A1FCF6F9793582E2FFE6B_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_575ED5D5599A1FCF6F9793582E2FFE6B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_A5B08BDDEE98876A464C93582E2F90CF" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_575ED5D5599A1FCF6F9793582E2FFE6B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GeneralBusinessMember" xlink:label="loc_us-gaap_GeneralBusinessMember_458EA3DD02696E6BC4A593582E2F9887" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardNameDomain_575ED5D5599A1FCF6F9793582E2FFE6B" xlink:to="loc_us-gaap_GeneralBusinessMember_458EA3DD02696E6BC4A593582E2F9887" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchMember" xlink:label="loc_us-gaap_ResearchMember_0B1F15C996EB5CAA454A93582E2F0700" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardNameDomain_575ED5D5599A1FCF6F9793582E2FFE6B" xlink:to="loc_us-gaap_ResearchMember_0B1F15C996EB5CAA454A93582E2F0700" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_E2C9DF5FDC5648AEF1D993582E2FED5F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_TaxCreditCarryforwardTable_B2C2EAED4AE128D95DD093582E2E2219" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_E2C9DF5FDC5648AEF1D993582E2FED5F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_BCDACC1F98F82E9B295093582E2F95DB_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_E2C9DF5FDC5648AEF1D993582E2FED5F" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_BCDACC1F98F82E9B295093582E2F95DB_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_BCDACC1F98F82E9B295093582E2F95DB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_E2C9DF5FDC5648AEF1D993582E2FED5F" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_BCDACC1F98F82E9B295093582E2F95DB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DomesticCountryMember" xlink:label="loc_us-gaap_DomesticCountryMember_9DEDB3D54181BB5FE67493582E2F70DE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_BCDACC1F98F82E9B295093582E2F95DB" xlink:to="loc_us-gaap_DomesticCountryMember_9DEDB3D54181BB5FE67493582E2F70DE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_6CD246E861244F85738493582E2F83F8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_BCDACC1F98F82E9B295093582E2F95DB" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_6CD246E861244F85738493582E2F83F8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignCountryMember" xlink:label="loc_us-gaap_ForeignCountryMember_DDEB699653B5CD04C67F93582E2F085C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_BCDACC1F98F82E9B295093582E2F95DB" xlink:to="loc_us-gaap_ForeignCountryMember_DDEB699653B5CD04C67F93582E2F085C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_AF2ABAA69DA13ADA5B8E93582E2FA19E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_TaxCreditCarryforwardTable_B2C2EAED4AE128D95DD093582E2E2219" xlink:to="loc_us-gaap_RangeAxis_AF2ABAA69DA13ADA5B8E93582E2FA19E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_47E465FC7CF20C97943793582E2F4D39_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_AF2ABAA69DA13ADA5B8E93582E2FA19E" xlink:to="loc_us-gaap_RangeMember_47E465FC7CF20C97943793582E2F4D39_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_47E465FC7CF20C97943793582E2F4D39" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_AF2ABAA69DA13ADA5B8E93582E2FA19E" xlink:to="loc_us-gaap_RangeMember_47E465FC7CF20C97943793582E2F4D39" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_6EE46D63C2C0CC8CD46A93582E2F9A75" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_47E465FC7CF20C97943793582E2F4D39" xlink:to="loc_us-gaap_MinimumMember_6EE46D63C2C0CC8CD46A93582E2F9A75" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_44B069374FAB56861C8393582E2F21C7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_47E465FC7CF20C97943793582E2F4D39" xlink:to="loc_us-gaap_MaximumMember_44B069374FAB56861C8393582E2F21C7" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Deferredtaxassetreclassification" xlink:label="loc_biib_Deferredtaxassetreclassification_71964BE76E996AC4AD4093582E2FA8D3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_biib_Deferredtaxassetreclassification_71964BE76E996AC4AD4093582E2FA8D3" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Deferredtaxliabilityreclassification" xlink:label="loc_biib_Deferredtaxliabilityreclassification_6A5C72435CE5DD42FC7B93582E3017EC" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_biib_Deferredtaxliabilityreclassification_6A5C72435CE5DD42FC7B93582E3017EC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Totaldeferredchargesandprepaidtaxes" xlink:label="loc_biib_Totaldeferredchargesandprepaidtaxes_EA60536622BE4B7406B293582E30371D" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_biib_Totaldeferredchargesandprepaidtaxes_EA60536622BE4B7406B293582E30371D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DeferredTaxLiabilityRecognized" xlink:label="loc_biib_DeferredTaxLiabilityRecognized_6B65C37FCB1267FE2A6193582E30DC4E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_biib_DeferredTaxLiabilityRecognized_6B65C37FCB1267FE2A6193582E30DC4E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxLiabilities" xlink:label="loc_us-gaap_DeferredTaxLiabilities_6860946B2A527EB06C4C93582E30FD61" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_us-gaap_DeferredTaxLiabilities_6860946B2A527EB06C4C93582E30FD61" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NetBenefitForPreviouslyUnrecognizedPosition" xlink:label="loc_biib_NetBenefitForPreviouslyUnrecognizedPosition_DD80BE9FFC638ED526F693582E3022D2" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_biib_NetBenefitForPreviouslyUnrecognizedPosition_DD80BE9FFC638ED526F693582E3022D2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLossCarryforwards" xlink:label="loc_us-gaap_OperatingLossCarryforwards_2BBAB2DD33D107A563DD93582E305729" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_us-gaap_OperatingLossCarryforwards_2BBAB2DD33D107A563DD93582E305729" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardAmount" xlink:label="loc_us-gaap_TaxCreditCarryforwardAmount_9A6C248890E77AD4A08D93582E30F101" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_us-gaap_TaxCreditCarryforwardAmount_9A6C248890E77AD4A08D93582E30F101" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal_75D824E7D34A092D851E93582E306473" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal_75D824E7D34A092D851E93582E306473" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EstimatedTaxLiabilityOnUndistributedEarningsOfForeignSubsidiaries" xlink:label="loc_biib_EstimatedTaxLiabilityOnUndistributedEarningsOfForeignSubsidiaries_5E0956CDF5D7981A279393582E307943" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_biib_EstimatedTaxLiabilityOnUndistributedEarningsOfForeignSubsidiaries_5E0956CDF5D7981A279393582E307943" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ExpenseForNonIncomeBasedStateTaxes" xlink:label="loc_biib_ExpenseForNonIncomeBasedStateTaxes_95D1E705EE20C1C39CE893582E3018DA" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_biib_ExpenseForNonIncomeBasedStateTaxes_95D1E705EE20C1C39CE893582E3018DA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices" xlink:label="loc_biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices_F872C17DCA650D15FD5493582E30CEE8" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_biib_ProposedDisallowanceByTaxAuthoritiesForPaymentForServices_F872C17DCA650D15FD5493582E30CEE8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions_3DC2598769BB2AB1F3C093582E3006B8" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardLineItems_BDD0A2C79602BB59D0C193582E2FC505" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions_3DC2598769BB2AB1F3C093582E3006B8" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/IncomeTaxesTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/Indebtedness" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IndebtednessDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_3057976B1A522670D18C38A351EB09DD" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_5433ADC4A08143DA0D6138A351EA686B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3057976B1A522670D18C38A351EB09DD" xlink:to="loc_us-gaap_DebtInstrumentTable_5433ADC4A08143DA0D6138A351EA686B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_B373CDA1FBCD5A8B70EF38A351EAF208" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5433ADC4A08143DA0D6138A351EA686B" xlink:to="loc_us-gaap_DebtInstrumentAxis_B373CDA1FBCD5A8B70EF38A351EAF208" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_B373CDA1FBCD5A8B70EF38A351EAF208" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_B373CDA1FBCD5A8B70EF38A351EAF208" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember" xlink:label="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_0085337D8828E67DD0B738A351EB7C2C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6" xlink:to="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_0085337D8828E67DD0B738A351EB7C2C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember" xlink:label="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_45FC4FB67ECBF578E4D838A351EB8A22" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6" xlink:to="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_45FC4FB67ECBF578E4D838A351EB8A22" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember" xlink:label="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_D55CF4FD3D9015E6E4A938A351EBBEEF" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6" xlink:to="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_D55CF4FD3D9015E6E4A938A351EBBEEF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" xlink:label="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_3824910742301AA8119A38A351EB4C4A" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6" xlink:to="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_3824910742301AA8119A38A351EB4C4A" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" xlink:label="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_6311357ECD8FFF8D30F238A351EBDDD6" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_808CADC0DADEE4DCB89538A351EAC0D6" xlink:to="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_6311357ECD8FFF8D30F238A351EBDDD6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_99BEF29C1FC75DF8F48C38A351EB2C1E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5433ADC4A08143DA0D6138A351EA686B" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_99BEF29C1FC75DF8F48C38A351EB2C1E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_911BE02EA1DEC33112F438A351EBC542_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_99BEF29C1FC75DF8F48C38A351EB2C1E" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_911BE02EA1DEC33112F438A351EBC542_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_911BE02EA1DEC33112F438A351EBC542" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_99BEF29C1FC75DF8F48C38A351EB2C1E" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_911BE02EA1DEC33112F438A351EBC542" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NotesPayableOtherPayablesMember" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_A604064A0E08FC5D609138A351EB6099" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_911BE02EA1DEC33112F438A351EBC542" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_A604064A0E08FC5D609138A351EB6099" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermDebtCurrentAbstract" xlink:label="loc_us-gaap_LongTermDebtCurrentAbstract_FCE66CB076C27EB71CED38A351EB3587" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3057976B1A522670D18C38A351EB09DD" xlink:to="loc_us-gaap_LongTermDebtCurrentAbstract_FCE66CB076C27EB71CED38A351EB3587" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NotesPayableCurrent" xlink:label="loc_us-gaap_NotesPayableCurrent_22F8762A23417647460B38A351EB50BE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtCurrentAbstract_FCE66CB076C27EB71CED38A351EB3587" xlink:to="loc_us-gaap_NotesPayableCurrent_22F8762A23417647460B38A351EB50BE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_FCE98FB685D4F612AE8B38A351EBA7EF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtCurrentAbstract_FCE66CB076C27EB71CED38A351EB3587" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_FCE98FB685D4F612AE8B38A351EBA7EF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtCurrent" xlink:label="loc_us-gaap_DebtCurrent_19E5A1ACA121766BC13B38A351ECDBFA" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtCurrentAbstract_FCE66CB076C27EB71CED38A351EB3587" xlink:to="loc_us-gaap_DebtCurrent_19E5A1ACA121766BC13B38A351ECDBFA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermDebtNoncurrentAbstract" xlink:label="loc_us-gaap_LongTermDebtNoncurrentAbstract_3B1F0C555BE913E113CE38A351EC4DB5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_3057976B1A522670D18C38A351EB09DD" xlink:to="loc_us-gaap_LongTermDebtNoncurrentAbstract_3B1F0C555BE913E113CE38A351EC4DB5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermNotesPayable" xlink:label="loc_us-gaap_LongTermNotesPayable_A0BD62888ED5C8C9B0F038A351ECFFA1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtNoncurrentAbstract_3B1F0C555BE913E113CE38A351EC4DB5" xlink:to="loc_us-gaap_LongTermNotesPayable_A0BD62888ED5C8C9B0F038A351ECFFA1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_26C4BB6C887D4582936A38A351ECFEC7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtNoncurrentAbstract_3B1F0C555BE913E113CE38A351EC4DB5" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_26C4BB6C887D4582936A38A351ECFEC7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermDebt" xlink:label="loc_us-gaap_LongTermDebt_8C310E52827A95BA75AE38A351EC474D" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtNoncurrentAbstract_3B1F0C555BE913E113CE38A351EC4DB5" xlink:to="loc_us-gaap_LongTermDebt_8C310E52827A95BA75AE38A351EC474D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/IndebtednessDetails1" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IndebtednessDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_EE972E59E1273D03648D93D5CA1E2D2D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_DebtInstrumentTable_EE972E59E1273D03648D93D5CA1E2D2D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0589111E6C67C78E40C993D5CA1FA6E3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_EE972E59E1273D03648D93D5CA1E2D2D" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0589111E6C67C78E40C993D5CA1FA6E3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_971902D456D20355A82D93D5CA1F7ED8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0589111E6C67C78E40C993D5CA1FA6E3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_971902D456D20355A82D93D5CA1F7ED8_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_971902D456D20355A82D93D5CA1F7ED8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0589111E6C67C78E40C993D5CA1FA6E3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_971902D456D20355A82D93D5CA1F7ED8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EisaiMember" xlink:label="loc_biib_EisaiMember_89F59F815B7C310F4BEF93D5CA1F2C71" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_971902D456D20355A82D93D5CA1F7ED8" xlink:to="loc_biib_EisaiMember_89F59F815B7C310F4BEF93D5CA1F2C71" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_EFBC1D8220BAA09FD98B93D5CA1F7987" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_EE972E59E1273D03648D93D5CA1E2D2D" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_EFBC1D8220BAA09FD98B93D5CA1F7987" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_85DABEB888190B9427AD93D5CA1F67E7_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_EFBC1D8220BAA09FD98B93D5CA1F7987" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_85DABEB888190B9427AD93D5CA1F67E7_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_85DABEB888190B9427AD93D5CA1F67E7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_EFBC1D8220BAA09FD98B93D5CA1F7987" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_85DABEB888190B9427AD93D5CA1F67E7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestRateSwapMember" xlink:label="loc_us-gaap_InterestRateSwapMember_D4288B47E7D0BA66F49693D5CA1F6CC5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_85DABEB888190B9427AD93D5CA1F67E7" xlink:to="loc_us-gaap_InterestRateSwapMember_D4288B47E7D0BA66F49693D5CA1F6CC5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="loc_us-gaap_DebtInstrumentAxis_860325E6506CC6C3943A93D5CA1FB26F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_EE972E59E1273D03648D93D5CA1E2D2D" xlink:to="loc_us-gaap_DebtInstrumentAxis_860325E6506CC6C3943A93D5CA1FB26F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_860325E6506CC6C3943A93D5CA1FB26F" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_860325E6506CC6C3943A93D5CA1FB26F" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember" xlink:label="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_59B31C7C9A3A736B503F93D5CA1F47BF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3" xlink:to="loc_biib_SeniorNotesTwoPointNinePercentDueTwentyTwentyMember_59B31C7C9A3A736B503F93D5CA1F47BF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember" xlink:label="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_8AFD80790EA34399A7BA93D5CA1FFD4F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3" xlink:to="loc_biib_SeniorNotesThreePointSixTwoFivePercentDueTwentyTwentyTwoMember_8AFD80790EA34399A7BA93D5CA1FFD4F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember" xlink:label="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_767C61E63B4356C4011693D5CA1F163E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3" xlink:to="loc_biib_SeniorNotesFourPointZeroFivePercentDueTwentyTwentyFiveMember_767C61E63B4356C4011693D5CA1F163E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember" xlink:label="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_DEA3CD82BFBCDA2F38BC93D5CA1F67F6" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3" xlink:to="loc_biib_SeniorNotesFivePointTwoPercentDueTwentyFortyFiveMember_DEA3CD82BFBCDA2F38BC93D5CA1F67F6" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember" xlink:label="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_7CE22C1F69C1430DA9BA93E5A6FBC5A4" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_5AC65E919D0FF2004BFA93D5CA1FFBD3" xlink:to="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_7CE22C1F69C1430DA9BA93E5A6FBC5A4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_FADA9BCA25F556DE271F93D5CA1FCF31" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_EE972E59E1273D03648D93D5CA1E2D2D" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_FADA9BCA25F556DE271F93D5CA1FCF31" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9493788F303D7B1E086E93D5CA1F73A4_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_FADA9BCA25F556DE271F93D5CA1FCF31" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9493788F303D7B1E086E93D5CA1F73A4_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9493788F303D7B1E086E93D5CA1F73A4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_FADA9BCA25F556DE271F93D5CA1FCF31" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9493788F303D7B1E086E93D5CA1F73A4" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember" xlink:label="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_697EC9037B3D3D886D3693D5CA1F5B18" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9493788F303D7B1E086E93D5CA1F73A4" xlink:to="loc_biib_SeniorNotesSixPointEightSevenFivePercentDueTwentyEighteenMember_697EC9037B3D3D886D3693D5CA1F5B18" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NotesPayableOtherPayablesMember" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_206CD114F4E55150E50993D5CA1F8118" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9493788F303D7B1E086E93D5CA1F73A4" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_206CD114F4E55150E50993D5CA1F8118" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_25F082A9E4EAC6D4D61293D5CA204531" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_25F082A9E4EAC6D4D61293D5CA204531" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_907BBD8298F21D66CE4C93D5CA20B3BB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_907BBD8298F21D66CE4C93D5CA20B3BB" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageParValueOfSeniorNotes" xlink:label="loc_biib_PercentageParValueOfSeniorNotes_CCD183A70709537A1B9D93D5CA20E814" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_biib_PercentageParValueOfSeniorNotes_CCD183A70709537A1B9D93D5CA20E814" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_12BE5EA91E95F29E8E3993D5CA202D58" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_12BE5EA91E95F29E8E3993D5CA202D58" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DebtInstrumentMakeWholeProvisionRedemptionPricePercentage" xlink:label="loc_biib_DebtInstrumentMakeWholeProvisionRedemptionPricePercentage_E11A275EB3E843B967F493D5CA202D6C" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_biib_DebtInstrumentMakeWholeProvisionRedemptionPricePercentage_E11A275EB3E843B967F493D5CA202D6C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtIssuanceCosts" xlink:label="loc_us-gaap_DebtIssuanceCosts_05703242317BF7C5967593D5CA203251" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_DebtIssuanceCosts_05703242317BF7C5967593D5CA203251" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_CF427481D02137E93E6D93D5CA202BC3" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_CF427481D02137E93E6D93D5CA202BC3" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageOfRedemptionOfNotes" xlink:label="loc_biib_PercentageOfRedemptionOfNotes_062780907191105FBFFA93D5CA20127C" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_biib_PercentageOfRedemptionOfNotes_062780907191105FBFFA93D5CA20127C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_IncreasedCarryingAmountOfSeniorNotes" xlink:label="loc_biib_IncreasedCarryingAmountOfSeniorNotes_5A186CE6CBA86C55210093D5CA205B87" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_biib_IncreasedCarryingAmountOfSeniorNotes_5A186CE6CBA86C55210093D5CA205B87" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DebtInstrumentUnamortizedInterestOnDebt" xlink:label="loc_biib_DebtInstrumentUnamortizedInterestOnDebt_86E476B5EBA1CC28D41B93D5CA209F6F" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_biib_DebtInstrumentUnamortizedInterestOnDebt_86E476B5EBA1CC28D41B93D5CA209F6F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongTermNotesPayable" xlink:label="loc_us-gaap_LongTermNotesPayable_FCAD46566DF217E0C03393D5CA2042CC" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_LongTermNotesPayable_FCAD46566DF217E0C03393D5CA2042CC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ParValueOfNotesPayable" xlink:label="loc_biib_ParValueOfNotesPayable_2CF1812C49CEF134DB3593D5CA2051E8" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_biib_ParValueOfNotesPayable_2CF1812C49CEF134DB3593D5CA2051E8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_C5DC1A416C14521A7A8A93D5CA2078C9" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_C5DC1A416C14521A7A8A93D5CA2078C9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LineOfCreditFacilityExpirationPeriod" xlink:label="loc_us-gaap_LineOfCreditFacilityExpirationPeriod_C79F9AE510112EA1414793D5CA20FF9F" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_LineOfCreditFacilityExpirationPeriod_C79F9AE510112EA1414793D5CA20FF9F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding" xlink:label="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_5D983F76332D71EF5F3B93D5CA207AC7" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_LineOfCreditFacilityFairValueOfAmountOutstanding_5D983F76332D71EF5F3B93D5CA207AC7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CapitalLeaseObligations" xlink:label="loc_us-gaap_CapitalLeaseObligations_17EE439A728C6C5E108793D5CA205A26" xlink:type="locator" />
    <definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BED839B79D6DD366DEC793D5CA206518" xlink:to="loc_us-gaap_CapitalLeaseObligations_17EE439A728C6C5E108793D5CA205A26" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/IndebtednessTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwill" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4F9063EB5D934C02A9FA93224CD4DC78" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4F9063EB5D934C02A9FA93224CD4DC78" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_35BF1C14C20E9719E31393224CD481C4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4F9063EB5D934C02A9FA93224CD4DC78" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_35BF1C14C20E9719E31393224CD481C4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0F3C0CCD51380D04CA1193224CD48751_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_35BF1C14C20E9719E31393224CD481C4" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0F3C0CCD51380D04CA1193224CD48751_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0F3C0CCD51380D04CA1193224CD48751" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_35BF1C14C20E9719E31393224CD481C4" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0F3C0CCD51380D04CA1193224CD48751" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ConvergencePharmaceuticalsMember" xlink:label="loc_biib_ConvergencePharmaceuticalsMember_78DBEEC366214FD9AF0A93224CD44D99" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0F3C0CCD51380D04CA1193224CD48751" xlink:to="loc_biib_ConvergencePharmaceuticalsMember_78DBEEC366214FD9AF0A93224CD44D99" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_1CB2691B3700402EF03693224CD4597E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4F9063EB5D934C02A9FA93224CD4DC78" xlink:to="loc_us-gaap_ProductOrServiceAxis_1CB2691B3700402EF03693224CD4597E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_E0C8DEEBB7E02923633193224CD493EF_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_1CB2691B3700402EF03693224CD4597E" xlink:to="loc_us-gaap_ProductsAndServicesDomain_E0C8DEEBB7E02923633193224CD493EF_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_E0C8DEEBB7E02923633193224CD493EF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_1CB2691B3700402EF03693224CD4597E" xlink:to="loc_us-gaap_ProductsAndServicesDomain_E0C8DEEBB7E02923633193224CD493EF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AVONEXMember" xlink:label="loc_biib_AVONEXMember_967F1C1E8B6DE26A06AB93224CD58816" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_E0C8DEEBB7E02923633193224CD493EF" xlink:to="loc_biib_AVONEXMember_967F1C1E8B6DE26A06AB93224CD58816" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TysabriProductMember" xlink:label="loc_biib_TysabriProductMember_3C18AE78F81848CE4FF693224CD5BB9D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_E0C8DEEBB7E02923633193224CD493EF" xlink:to="loc_biib_TysabriProductMember_3C18AE78F81848CE4FF693224CD5BB9D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_631D0E0BAEC2B363B14693224CD5D7F9" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4F9063EB5D934C02A9FA93224CD4DC78" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_631D0E0BAEC2B363B14693224CD5D7F9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_EBEA22071E45F6D0A56993224CD50159_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_631D0E0BAEC2B363B14693224CD5D7F9" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_EBEA22071E45F6D0A56993224CD50159_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_EBEA22071E45F6D0A56993224CD50159" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_631D0E0BAEC2B363B14693224CD5D7F9" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_EBEA22071E45F6D0A56993224CD50159" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InProcessResearchAndDevelopmentMember" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_EE3FBB42D063C2CAF29193224CD53EDC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_EBEA22071E45F6D0A56993224CD50159" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_EE3FBB42D063C2CAF29193224CD53EDC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TrademarksAndTradeNamesMember" xlink:label="loc_us-gaap_TrademarksAndTradeNamesMember_F579532BF40DE32380B693224CDADD1B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_EBEA22071E45F6D0A56993224CD50159" xlink:to="loc_us-gaap_TrademarksAndTradeNamesMember_F579532BF40DE32380B693224CDADD1B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_A3EF29F06427E7FFB3F093224CDAA2AC" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4F9063EB5D934C02A9FA93224CD4DC78" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_A3EF29F06427E7FFB3F093224CDAA2AC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6448862C0974759741B993224CDA8DC8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_A3EF29F06427E7FFB3F093224CDAA2AC" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6448862C0974759741B993224CDA8DC8_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6448862C0974759741B993224CDA8DC8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_A3EF29F06427E7FFB3F093224CDAA2AC" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6448862C0974759741B993224CDA8DC8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_OutLicensedPatentsMember" xlink:label="loc_biib_OutLicensedPatentsMember_029BDACC881DC6EEA47093224CDAD84A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6448862C0974759741B993224CDA8DC8" xlink:to="loc_biib_OutLicensedPatentsMember_029BDACC881DC6EEA47093224CDAD84A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_6253A9E80E7FC0196DD993224CDAB87C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6448862C0974759741B993224CDA8DC8" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_6253A9E80E7FC0196DD993224CDAB87C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_InLicensedPatentsMember" xlink:label="loc_biib_InLicensedPatentsMember_541EEB822EE2F808604893224CDA2BFB" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_6448862C0974759741B993224CDA8DC8" xlink:to="loc_biib_InLicensedPatentsMember_541EEB822EE2F808604893224CDA2BFB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_F484C7AB99B3F86138D593224CDA6C84" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4F9063EB5D934C02A9FA93224CD4DC78" xlink:to="loc_us-gaap_RangeAxis_F484C7AB99B3F86138D593224CDA6C84" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_73C1DCED6391BC31BBF893224CDBF21B_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_F484C7AB99B3F86138D593224CDA6C84" xlink:to="loc_us-gaap_RangeMember_73C1DCED6391BC31BBF893224CDBF21B_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_73C1DCED6391BC31BBF893224CDBF21B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_F484C7AB99B3F86138D593224CDA6C84" xlink:to="loc_us-gaap_RangeMember_73C1DCED6391BC31BBF893224CDBF21B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_C2B1ED635F518B2F805493224CDB7841" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_73C1DCED6391BC31BBF893224CDBF21B" xlink:to="loc_us-gaap_MinimumMember_C2B1ED635F518B2F805493224CDB7841" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_EC541F2B77398883C1B993224CDB19D0" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_73C1DCED6391BC31BBF893224CDBF21B" xlink:to="loc_us-gaap_MaximumMember_EC541F2B77398883C1B993224CDB19D0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_550A17DC156B1EA5351493224CDB34A3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:to="loc_us-gaap_Goodwill_550A17DC156B1EA5351493224CDB34A3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_6AAA18418CECF4AC42FE93224CDB60C5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_6AAA18418CECF4AC42FE93224CDB60C5" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_IndefiniteLivedIntangibleAssetsUsefulLife" xlink:label="loc_biib_IndefiniteLivedIntangibleAssetsUsefulLife_EE6B94DAA428326B583493224CDB4832" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:to="loc_biib_IndefiniteLivedIntangibleAssetsUsefulLife_EE6B94DAA428326B583493224CDB4832" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_67E1D80199CD61F45DDE93224CDBD485" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_67E1D80199CD61F45DDE93224CDBD485" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_580BAF5F95D2553B699E93224CDB2C1D" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_580BAF5F95D2553B699E93224CDB2C1D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_FE91F4D170521E3001BE93224CDB46F5" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_FE91F4D170521E3001BE93224CDB46F5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_62401F5858547C75BE6993224CDBEEA6" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_62401F5858547C75BE6993224CDBEEA6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_7FDBD5F4A46A832C49B293224CDB6FBC" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_7FDBD5F4A46A832C49B293224CDB6FBC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_9447849594F2FEFEB52A93224CDB0E46" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IntangibleAssetsNetExcludingGoodwillAbstract_22C90D6029C542C0215C93224CDB4ABD" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_9447849594F2FEFEB52A93224CDB0E46" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract" xlink:label="loc_biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract_E0CFE1D677D4628385DD93224CDBCDA5" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:to="loc_biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract_E0CFE1D677D4628385DD93224CDBCDA5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths_C23F119A34CE854FFD0893224CDB2BEB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract_E0CFE1D677D4628385DD93224CDBCDA5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths_C23F119A34CE854FFD0893224CDB2BEB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo_0BF8322266308215EE8593224CDBDCC2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract_E0CFE1D677D4628385DD93224CDBCDA5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo_0BF8322266308215EE8593224CDBDCC2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree_3803DBFEB22501377C4C93224CDC7239" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract_E0CFE1D677D4628385DD93224CDBCDA5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree_3803DBFEB22501377C4C93224CDC7239" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour_39EF1AE3183A43A1795293224CDCB194" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract_E0CFE1D677D4628385DD93224CDBCDA5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour_39EF1AE3183A43A1795293224CDCB194" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive_D4C188C6827958A0554693224CDC1327" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract_E0CFE1D677D4628385DD93224CDBCDA5" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive_D4C188C6827958A0554693224CDC1327" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Finitelivedintangibleassetsfutureamortization" xlink:label="loc_biib_Finitelivedintangibleassetsfutureamortization_1CE2EFA9D1B450CBAFA793224CDC9A14" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_IntangibleAssetsAndGoodwillAdditionalTextualAbstract_E0CFE1D677D4628385DD93224CDBCDA5" xlink:to="loc_biib_Finitelivedintangibleassetsfutureamortization_1CE2EFA9D1B450CBAFA793224CDC9A14" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_D901030EFCADAFC40D1F93224CDC563F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_D901030EFCADAFC40D1F93224CDC563F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ImpairmentOfIntangibleAssetsFinitelived" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_9F7308450AECCEBFC24C93224CDCB2C2" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsFinitelived_9F7308450AECCEBFC24C93224CDCB2C2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill" xlink:label="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_73600D0315A83E03C4F193224CDC9F24" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:to="loc_us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill_73600D0315A83E03C4F193224CDC9F24" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_DDDE86A3DE7CC7DAD0F493224CDCFC0D" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_DD0FB3BAB9D03498142F93224CDBA5FA" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_DDDE86A3DE7CC7DAD0F493224CDCFC0D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="loc_us-gaap_GoodwillLineItems_6937FB604D575D7DA65693224CACA0C2" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfGoodwillTable" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_5DFEA886036FE17FC47193224CAB9C9B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_6937FB604D575D7DA65693224CACA0C2" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_5DFEA886036FE17FC47193224CAB9C9B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_6137E3E4E4C5197F9D9693224CAB9CF7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_5DFEA886036FE17FC47193224CAB9C9B" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_6137E3E4E4C5197F9D9693224CAB9CF7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F0DB72D01F24E9E1496A93224CAB8541_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_6137E3E4E4C5197F9D9693224CAB9CF7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F0DB72D01F24E9E1496A93224CAB8541_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F0DB72D01F24E9E1496A93224CAB8541" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_6137E3E4E4C5197F9D9693224CAB9CF7" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F0DB72D01F24E9E1496A93224CAB8541" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FumapharmAgMember" xlink:label="loc_biib_FumapharmAgMember_17F00435A67785A555ED93224CAC203F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F0DB72D01F24E9E1496A93224CAB8541" xlink:to="loc_biib_FumapharmAgMember_17F00435A67785A555ED93224CAC203F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ConvergencePharmaceuticalsMember" xlink:label="loc_biib_ConvergencePharmaceuticalsMember_3A2855E2A30F57572D5893224CAC2502" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F0DB72D01F24E9E1496A93224CAB8541" xlink:to="loc_biib_ConvergencePharmaceuticalsMember_3A2855E2A30F57572D5893224CAC2502" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_F625A013400BE92F078393224CAC107E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_5DFEA886036FE17FC47193224CAB9C9B" xlink:to="loc_us-gaap_ProductOrServiceAxis_F625A013400BE92F078393224CAC107E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_2BE34A7DEA59CAA7FE5193224CAC1CC9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_F625A013400BE92F078393224CAC107E" xlink:to="loc_us-gaap_ProductsAndServicesDomain_2BE34A7DEA59CAA7FE5193224CAC1CC9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_2BE34A7DEA59CAA7FE5193224CAC1CC9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_F625A013400BE92F078393224CAC107E" xlink:to="loc_us-gaap_ProductsAndServicesDomain_2BE34A7DEA59CAA7FE5193224CAC1CC9" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TecfideraMember" xlink:label="loc_biib_TecfideraMember_362DBCE9D8CB4B86AB0493224CAC2725" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_2BE34A7DEA59CAA7FE5193224CAC1CC9" xlink:to="loc_biib_TecfideraMember_362DBCE9D8CB4B86AB0493224CAC2725" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_230C835D6A19260E000293224CAC7E07" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_6937FB604D575D7DA65693224CACA0C2" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_230C835D6A19260E000293224CAC7E07" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GoodwillImpairedAccumulatedImpairmentLoss" xlink:label="loc_us-gaap_GoodwillImpairedAccumulatedImpairmentLoss_71B1D94AC2E8E3844CAF93224CB11877" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_6937FB604D575D7DA65693224CACA0C2" xlink:to="loc_us-gaap_GoodwillImpairedAccumulatedImpairmentLoss_71B1D94AC2E8E3844CAF93224CB11877" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/IntangibleAssetsAndGoodwillTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/Inventory" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/InventoryDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/InventoryDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InventoryLineItems" xlink:label="loc_us-gaap_InventoryLineItems_A319C20CA67DB99B8C0F93224CFEEAC5" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AirlineProductsAndServicesTable" xlink:label="loc_us-gaap_AirlineProductsAndServicesTable_63832FB8ECD162BE35FB93224CFDA2AD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InventoryLineItems_A319C20CA67DB99B8C0F93224CFEEAC5" xlink:to="loc_us-gaap_AirlineProductsAndServicesTable_63832FB8ECD162BE35FB93224CFDA2AD" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_BDDBD00F820312A6FEAB93224CFE1142" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AirlineProductsAndServicesTable_63832FB8ECD162BE35FB93224CFDA2AD" xlink:to="loc_us-gaap_ProductOrServiceAxis_BDDBD00F820312A6FEAB93224CFE1142" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_9260D1A09B45C728FDEB93224CFED462_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_BDDBD00F820312A6FEAB93224CFE1142" xlink:to="loc_us-gaap_ProductsAndServicesDomain_9260D1A09B45C728FDEB93224CFED462_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_9260D1A09B45C728FDEB93224CFED462" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_BDDBD00F820312A6FEAB93224CFE1142" xlink:to="loc_us-gaap_ProductsAndServicesDomain_9260D1A09B45C728FDEB93224CFED462" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ZINBRYTAMember" xlink:label="loc_biib_ZINBRYTAMember_51B9B2FDC7A334F8F90093224CFE83EE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_9260D1A09B45C728FDEB93224CFED462" xlink:to="loc_biib_ZINBRYTAMember_51B9B2FDC7A334F8F90093224CFE83EE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_BENEPALIMember" xlink:label="loc_biib_BENEPALIMember_F1CDA2889F89AFDEA4F39348C760D985" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_9260D1A09B45C728FDEB93224CFED462" xlink:to="loc_biib_BENEPALIMember_F1CDA2889F89AFDEA4F39348C760D985" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FLIXABIMember" xlink:label="loc_biib_FLIXABIMember_7ED2C1D896193DA9B4DE9349E8DD90CE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_9260D1A09B45C728FDEB93224CFED462" xlink:to="loc_biib_FLIXABIMember_7ED2C1D896193DA9B4DE9349E8DD90CE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Preapprovalinventory" xlink:label="loc_biib_Preapprovalinventory_C2E4723CC87A5104ACE593224CFEEB95" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_A319C20CA67DB99B8C0F93224CFEEAC5" xlink:to="loc_biib_Preapprovalinventory_C2E4723CC87A5104ACE593224CFEEB95" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/InventoryTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/InvestmentsInVariableInterestEntities" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/InvestmentsInVariableInterestEntitiesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VariableInterestEntityLineItems" xlink:label="loc_us-gaap_VariableInterestEntityLineItems_CF0C9258BAA90ED184D993582E12A7E6" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable" xlink:label="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_ACAA23A49FE96951D3D993582E116794" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_CF0C9258BAA90ED184D993582E12A7E6" xlink:to="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_ACAA23A49FE96951D3D993582E116794" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis" xlink:label="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_95BC0FB58538605DFA5893582E11C4A5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_ACAA23A49FE96951D3D993582E116794" xlink:to="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_95BC0FB58538605DFA5893582E11C4A5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ClassificationOfVariableInterestEntityDomain" xlink:label="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_EDB337D64A68833E396F93582E11B77D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_95BC0FB58538605DFA5893582E11C4A5" xlink:to="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_EDB337D64A68833E396F93582E11B77D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ClassificationOfVariableInterestEntityDomain" xlink:label="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_EDB337D64A68833E396F93582E11B77D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_95BC0FB58538605DFA5893582E11C4A5" xlink:to="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_EDB337D64A68833E396F93582E11B77D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NeurimmuneMember" xlink:label="loc_biib_NeurimmuneMember_E451959490AEA52B965C93582E12DEEE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_EDB337D64A68833E396F93582E11B77D" xlink:to="loc_biib_NeurimmuneMember_E451959490AEA52B965C93582E12DEEE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RodinTherapeuticsMember" xlink:label="loc_biib_RodinTherapeuticsMember_67FD71452ABDB315E16293582E12A9CD" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_EDB337D64A68833E396F93582E11B77D" xlink:to="loc_biib_RodinTherapeuticsMember_67FD71452ABDB315E16293582E12A9CD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_D0E71C59F74910FA3AB893582E12B471" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_CF0C9258BAA90ED184D993582E12A7E6" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_D0E71C59F74910FA3AB893582E12B471" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_InvestmentInVariableInterestEntitiesTextualAbstract" xlink:label="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_CF0C9258BAA90ED184D993582E12A7E6" xlink:to="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RemainingPotentialDevelopmentMilestonePaymentsAndRoyaltiesOnCommercialSalesUnderTermsOfCollaborationAgreement" xlink:label="loc_biib_RemainingPotentialDevelopmentMilestonePaymentsAndRoyaltiesOnCommercialSalesUnderTermsOfCollaborationAgreement_7AC122855F0F1564806893582E12EDBF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:to="loc_biib_RemainingPotentialDevelopmentMilestonePaymentsAndRoyaltiesOnCommercialSalesUnderTermsOfCollaborationAgreement_7AC122855F0F1564806893582E12EDBF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ResearchAndDevelopmentCostInSupportOfCollaborationAgreement" xlink:label="loc_biib_ResearchAndDevelopmentCostInSupportOfCollaborationAgreement_E18D82267E8A1717B8D793582E1216D6" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:to="loc_biib_ResearchAndDevelopmentCostInSupportOfCollaborationAgreement_E18D82267E8A1717B8D793582E1216D6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates" xlink:label="loc_us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates_4161E02A52751C889D4F93582E12C6E0" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:to="loc_us-gaap_PaymentsToAcquireInterestInSubsidiariesAndAffiliates_4161E02A52751C889D4F93582E12C6E0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitment" xlink:label="loc_us-gaap_OtherCommitment_C00A27608CF68B15FA9093582E128A66" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:to="loc_us-gaap_OtherCommitment_C00A27608CF68B15FA9093582E128A66" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentInProcess" xlink:label="loc_us-gaap_ResearchAndDevelopmentInProcess_FD50110CA21A4198118293582E12CD34" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:to="loc_us-gaap_ResearchAndDevelopmentInProcess_FD50110CA21A4198118293582E12CD34" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncontrollingInterestInVariableInterestEntity" xlink:label="loc_us-gaap_NoncontrollingInterestInVariableInterestEntity_2F033F03E873A4E04A6D93582E120BB6" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:to="loc_us-gaap_NoncontrollingInterestInVariableInterestEntity_2F033F03E873A4E04A6D93582E120BB6" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities" xlink:label="loc_biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities_0F6EA27E012A1106F31093582E12B6B9" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_InvestmentInVariableInterestEntitiesTextualAbstract_9CBED96917F637D416B593582E12BF1B" xlink:to="loc_biib_InvestmentInBiotechnologyCompaniesThatAreDeterminedToBeUnconsolidatedVariableInterestEntities_0F6EA27E012A1106F31093582E12B6B9" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/Litigation" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/LitigationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LossContingenciesLineItems" xlink:label="loc_us-gaap_LossContingenciesLineItems_6681F07F45EA49BE213938A352087500" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LossContingenciesTable" xlink:label="loc_us-gaap_LossContingenciesTable_222F11C1B7136752E23638A352086ACA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_6681F07F45EA49BE213938A352087500" xlink:to="loc_us-gaap_LossContingenciesTable_222F11C1B7136752E23638A352086ACA" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_E6609F50E3182813128638A35208D6B4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_222F11C1B7136752E23638A352086ACA" xlink:to="loc_us-gaap_StatementGeographicalAxis_E6609F50E3182813128638A35208D6B4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_D6765D53F81CCC261F9038A352085D92_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_E6609F50E3182813128638A35208D6B4" xlink:to="loc_us-gaap_SegmentGeographicalDomain_D6765D53F81CCC261F9038A352085D92_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_D6765D53F81CCC261F9038A352085D92" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_E6609F50E3182813128638A35208D6B4" xlink:to="loc_us-gaap_SegmentGeographicalDomain_D6765D53F81CCC261F9038A352085D92" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_IT" xlink:label="loc_country_IT_7A40A3C9EF397CD27DBB38A3520882F1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_D6765D53F81CCC261F9038A352085D92" xlink:to="loc_country_IT_7A40A3C9EF397CD27DBB38A3520882F1" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ReimbursementLimitStatedInResolution" xlink:label="loc_biib_ReimbursementLimitStatedInResolution_1A30F955400C20011CC238A352087B44" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_6681F07F45EA49BE213938A352087500" xlink:to="loc_biib_ReimbursementLimitStatedInResolution_1A30F955400C20011CC238A352087B44" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetail" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_F1501C35B3A715D8DC7193582E229042" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_02D32A0E4C21C40BDAA993582E2291E1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_F1501C35B3A715D8DC7193582E229042" xlink:to="loc_us-gaap_StatementTable_02D32A0E4C21C40BDAA993582E2291E1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_C771E0094C40EA166EAC93582E22B326" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_02D32A0E4C21C40BDAA993582E2291E1" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_C771E0094C40EA166EAC93582E22B326" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_0DCA0D6CC1F077A8C5EF93582E227B95_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_C771E0094C40EA166EAC93582E22B326" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_0DCA0D6CC1F077A8C5EF93582E227B95_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_0DCA0D6CC1F077A8C5EF93582E227B95" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_C771E0094C40EA166EAC93582E22B326" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_0DCA0D6CC1F077A8C5EF93582E227B95" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCurrentLiabilitiesMember" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_88964426E76164C547D793582E22DA3E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_0DCA0D6CC1F077A8C5EF93582E227B95" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_88964426E76164C547D793582E22DA3E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ValuationAllowancesAndReservesBalance" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesBalance_1A3F21360DF7A429BC4993582E2211A8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_F1501C35B3A715D8DC7193582E229042" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesBalance_1A3F21360DF7A429BC4993582E2211A8" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_854C756DFB8ABBD1471F93582E1AAA5E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_854C756DFB8ABBD1471F93582E1AAA5E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_CECA1ACFBEFB6A00B7DF93582E1AD795" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_854C756DFB8ABBD1471F93582E1AAA5E" xlink:to="loc_us-gaap_StatementGeographicalAxis_CECA1ACFBEFB6A00B7DF93582E1AD795" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_BD4C99E8B1F5084D352F93582E1A3421_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_CECA1ACFBEFB6A00B7DF93582E1AD795" xlink:to="loc_us-gaap_SegmentGeographicalDomain_BD4C99E8B1F5084D352F93582E1A3421_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_BD4C99E8B1F5084D352F93582E1A3421" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_CECA1ACFBEFB6A00B7DF93582E1AD795" xlink:to="loc_us-gaap_SegmentGeographicalDomain_BD4C99E8B1F5084D352F93582E1A3421" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_IT" xlink:label="loc_country_IT_538F9BEC69198083C6EC93582E1A3640" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_BD4C99E8B1F5084D352F93582E1A3421" xlink:to="loc_country_IT_538F9BEC69198083C6EC93582E1A3640" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_115EE47B89E8256D1F0793582E1A0F25" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_854C756DFB8ABBD1471F93582E1AAA5E" xlink:to="loc_us-gaap_ProductOrServiceAxis_115EE47B89E8256D1F0793582E1A0F25" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_D13EF9936227009823C693582E1BCAE2_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_115EE47B89E8256D1F0793582E1A0F25" xlink:to="loc_us-gaap_ProductsAndServicesDomain_D13EF9936227009823C693582E1BCAE2_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_D13EF9936227009823C693582E1BCAE2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_115EE47B89E8256D1F0793582E1A0F25" xlink:to="loc_us-gaap_ProductsAndServicesDomain_D13EF9936227009823C693582E1BCAE2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TecfideraMember" xlink:label="loc_biib_TecfideraMember_299D3CDE1A69CD4A0FBC93582E1BF245" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_D13EF9936227009823C693582E1BCAE2" xlink:to="loc_biib_TecfideraMember_299D3CDE1A69CD4A0FBC93582E1BF245" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CumulativeSalesLevelAxis" xlink:label="loc_biib_CumulativeSalesLevelAxis_A7C4C764F6D4CD8D1D2093582E1B15DE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_854C756DFB8ABBD1471F93582E1AAA5E" xlink:to="loc_biib_CumulativeSalesLevelAxis_A7C4C764F6D4CD8D1D2093582E1B15DE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CumulativeSalesLevelDomain" xlink:label="loc_biib_CumulativeSalesLevelDomain_81C67463153D48212EA393582E1B86CE_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_CumulativeSalesLevelAxis_A7C4C764F6D4CD8D1D2093582E1B15DE" xlink:to="loc_biib_CumulativeSalesLevelDomain_81C67463153D48212EA393582E1B86CE_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CumulativeSalesLevelDomain" xlink:label="loc_biib_CumulativeSalesLevelDomain_81C67463153D48212EA393582E1B86CE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_CumulativeSalesLevelAxis_A7C4C764F6D4CD8D1D2093582E1B15DE" xlink:to="loc_biib_CumulativeSalesLevelDomain_81C67463153D48212EA393582E1B86CE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SevenbillionMember" xlink:label="loc_biib_SevenbillionMember_567169C21EE3568D5BFA93582E1BBB75" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CumulativeSalesLevelDomain_81C67463153D48212EA393582E1B86CE" xlink:to="loc_biib_SevenbillionMember_567169C21EE3568D5BFA93582E1BBB75" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_71775D7C5777969FB1C093582E1BB749" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_854C756DFB8ABBD1471F93582E1AAA5E" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_71775D7C5777969FB1C093582E1BB749" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_44B3BB169B8FBE0C617393582E1B1347_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_71775D7C5777969FB1C093582E1BB749" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_44B3BB169B8FBE0C617393582E1B1347_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_44B3BB169B8FBE0C617393582E1B1347" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_71775D7C5777969FB1C093582E1BB749" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_44B3BB169B8FBE0C617393582E1B1347" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FumapharmAgMember" xlink:label="loc_biib_FumapharmAgMember_E8FE24E5BAE042A22F0D93582E1B825F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_44B3BB169B8FBE0C617393582E1B1347" xlink:to="loc_biib_FumapharmAgMember_E8FE24E5BAE042A22F0D93582E1B825F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SolothurnMember" xlink:label="loc_biib_SolothurnMember_EC7DDEC790CBD9C1540A93A5AB807D46" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_44B3BB169B8FBE0C617393582E1B1347" xlink:to="loc_biib_SolothurnMember_EC7DDEC790CBD9C1540A93A5AB807D46" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ConvergencePharmaceuticalsMember" xlink:label="loc_biib_ConvergencePharmaceuticalsMember_C32F9DFF6253F09DB7AC93582E1BC9C7" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_44B3BB169B8FBE0C617393582E1B1347" xlink:to="loc_biib_ConvergencePharmaceuticalsMember_C32F9DFF6253F09DB7AC93582E1BC9C7" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationAxis" xlink:label="loc_biib_FacilityLocationAxis_C5362AF692EB0190BB2F93582E1B6084" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_854C756DFB8ABBD1471F93582E1AAA5E" xlink:to="loc_biib_FacilityLocationAxis_C5362AF692EB0190BB2F93582E1B6084" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationDomain" xlink:label="loc_biib_FacilityLocationDomain_D366C70A71587103385893582E1B3AD8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_FacilityLocationAxis_C5362AF692EB0190BB2F93582E1B6084" xlink:to="loc_biib_FacilityLocationDomain_D366C70A71587103385893582E1B3AD8_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationDomain" xlink:label="loc_biib_FacilityLocationDomain_D366C70A71587103385893582E1B3AD8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_FacilityLocationAxis_C5362AF692EB0190BB2F93582E1B6084" xlink:to="loc_biib_FacilityLocationDomain_D366C70A71587103385893582E1B3AD8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CambridgeLeasesMember" xlink:label="loc_biib_CambridgeLeasesMember_68646E544EEDF91D34DF93582E1B92AB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_FacilityLocationDomain_D366C70A71587103385893582E1B3AD8" xlink:to="loc_biib_CambridgeLeasesMember_68646E544EEDF91D34DF93582E1B92AB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_E5F49049122DC812941193582E1B2405" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_E5F49049122DC812941193582E1B2405" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CumulativeSalesLevel" xlink:label="loc_biib_CumulativeSalesLevel_401908750E8A9FABA4A693582E1BD155" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_biib_CumulativeSalesLevel_401908750E8A9FABA4A693582E1BD155" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherLiabilitiesCurrent" xlink:label="loc_us-gaap_OtherLiabilitiesCurrent_DFEB40D38CAA2B365F9B93A5D37497DD" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_OtherLiabilitiesCurrent_DFEB40D38CAA2B365F9B93A5D37497DD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_FA5731EB42A753379F0A93582E1C51AF" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases_FA5731EB42A753379F0A93582E1C51AF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DerecognitionOfConstructionInProgressAssets" xlink:label="loc_biib_DerecognitionOfConstructionInProgressAssets_F0271AC9C526803ED86493582E1C5DEC" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_biib_DerecognitionOfConstructionInProgressAssets_F0271AC9C526803ED86493582E1C5DEC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PrepaidTaxes" xlink:label="loc_us-gaap_PrepaidTaxes_3B74A10E7B2101F964E193A7016DA739" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_PrepaidTaxes_3B74A10E7B2101F964E193A7016DA739" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LossContingencyEstimateOfPossibleLoss" xlink:label="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_17F8F9CCD5C2093E03C493582E1C4B0D" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_17F8F9CCD5C2093E03C493582E1C4B0D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LossContingencyAccrualAtCarryingValue" xlink:label="loc_us-gaap_LossContingencyAccrualAtCarryingValue_C15660B4201AC26AAC9893582E1C052C" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_LossContingencyAccrualAtCarryingValue_C15660B4201AC26AAC9893582E1C052C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_LossContingencyAdjustmentRelatedToClaimAmountPercent" xlink:label="loc_biib_LossContingencyAdjustmentRelatedToClaimAmountPercent_C553290695048CF5AA6F93582E1CB1D5" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_biib_LossContingencyAdjustmentRelatedToClaimAmountPercent_C553290695048CF5AA6F93582E1CB1D5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_202C99E7DD9851F899EE93582E1CD17E" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_202C99E7DD9851F899EE93582E1CD17E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_75B5C5DFFA579B5507F293A8926B620D" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_C47B1C3089049980425F93582E1B51FA" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_75B5C5DFFA579B5507F293A8926B620D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/OtherConsolidatedFinancialStatementDetailTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/PropertyPlantAndEquipment" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/PropertyPlantAndEquipmentDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/PropertyPlantAndEquipmentDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_526149A3AD8A06D3698493582EBFD06E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_526149A3AD8A06D3698493582EBFD06E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_B9739312139686999FA893582EC036EB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_526149A3AD8A06D3698493582EBFD06E" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_B9739312139686999FA893582EC036EB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_12E3CEB31452D6B5582B93582EC0A160_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_B9739312139686999FA893582EC036EB" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_12E3CEB31452D6B5582B93582EC0A160_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_12E3CEB31452D6B5582B93582EC0A160" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_B9739312139686999FA893582EC036EB" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_12E3CEB31452D6B5582B93582EC0A160" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LandMember" xlink:label="loc_us-gaap_LandMember_0B6E8E89E7C515A8D1B693582EC09A02" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_12E3CEB31452D6B5582B93582EC0A160" xlink:to="loc_us-gaap_LandMember_0B6E8E89E7C515A8D1B693582EC09A02" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_604B2B699C19D89521E393582EC0F757" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_526149A3AD8A06D3698493582EBFD06E" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_604B2B699C19D89521E393582EC0F757" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_03A68F34153EEF2BC6C993582EC0FD48_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_604B2B699C19D89521E393582EC0F757" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_03A68F34153EEF2BC6C993582EC0FD48_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_03A68F34153EEF2BC6C993582EC0FD48" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_604B2B699C19D89521E393582EC0F757" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_03A68F34153EEF2BC6C993582EC0FD48" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EisaiMember" xlink:label="loc_biib_EisaiMember_18C747C4DBA60D6AE48A93582EC0DD50" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_03A68F34153EEF2BC6C993582EC0FD48" xlink:to="loc_biib_EisaiMember_18C747C4DBA60D6AE48A93582EC0DD50" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationAxis" xlink:label="loc_biib_FacilityLocationAxis_FB29CD8146A6A876232193582EC05A13" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_526149A3AD8A06D3698493582EBFD06E" xlink:to="loc_biib_FacilityLocationAxis_FB29CD8146A6A876232193582EC05A13" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationDomain" xlink:label="loc_biib_FacilityLocationDomain_1CD8E2999E95A83BAC0C93582EC0204D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_FacilityLocationAxis_FB29CD8146A6A876232193582EC05A13" xlink:to="loc_biib_FacilityLocationDomain_1CD8E2999E95A83BAC0C93582EC0204D_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FacilityLocationDomain" xlink:label="loc_biib_FacilityLocationDomain_1CD8E2999E95A83BAC0C93582EC0204D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_FacilityLocationAxis_FB29CD8146A6A876232193582EC05A13" xlink:to="loc_biib_FacilityLocationDomain_1CD8E2999E95A83BAC0C93582EC0204D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SolothurnMember" xlink:label="loc_biib_SolothurnMember_653BCD04F283A92AF96893582EC05621" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_FacilityLocationDomain_1CD8E2999E95A83BAC0C93582EC0204D" xlink:to="loc_biib_SolothurnMember_653BCD04F283A92AF96893582EC05621" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CambridgeLeasesMember" xlink:label="loc_biib_CambridgeLeasesMember_0F9D636F62E40B76FC2D93582EC0A22B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_FacilityLocationDomain_1CD8E2999E95A83BAC0C93582EC0204D" xlink:to="loc_biib_CambridgeLeasesMember_0F9D636F62E40B76FC2D93582EC0A22B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Depreciation" xlink:label="loc_us-gaap_Depreciation_50379A40152691BDA69F93582EC0FC45" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_us-gaap_Depreciation_50379A40152691BDA69F93582EC0FC45" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestCostsCapitalized" xlink:label="loc_us-gaap_InterestCostsCapitalized_96C5DC5019D5689310B093582EC0EED1" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_us-gaap_InterestCostsCapitalized_96C5DC5019D5689310B093582EC0EED1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentAdditions" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAdditions_2760805C27537E3F935E93582EC04F73" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAdditions_2760805C27537E3F935E93582EC04F73" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConstructionInProgressGross" xlink:label="loc_us-gaap_ConstructionInProgressGross_4332A13B17E127473DD6936629FA14AA" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_us-gaap_ConstructionInProgressGross_4332A13B17E127473DD6936629FA14AA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract" xlink:label="loc_us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract_E8FA4A619E63B8DA1C8693582EC0AC40" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract_E8FA4A619E63B8DA1C8693582EC0AC40" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TermOfLease" xlink:label="loc_biib_TermOfLease_27CF5E8D36D42AA6B9D493582EC1BA5E" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_biib_TermOfLease_27CF5E8D36D42AA6B9D493582EC1BA5E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CapitalLeaseObligations" xlink:label="loc_us-gaap_CapitalLeaseObligations_573043FDF49FC9F474AF93582EC14D0D" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_us-gaap_CapitalLeaseObligations_573043FDF49FC9F474AF93582EC14D0D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Chargesrecognizedtovacatebuilding" xlink:label="loc_biib_Chargesrecognizedtovacatebuilding_CF1F1466AA378E4E574293582EC15DA5" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_78463B4BD7E3FA4AC96193582EC07344" xlink:to="loc_biib_Chargesrecognizedtovacatebuilding_CF1F1466AA378E4E574293582EC15DA5" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/PropertyPlantAndEquipmentPropertyPlantAndEquipmentDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_5D5E26F77EEAC39FFD2B38A356E2D447" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3B3E4E4C503D9CA450BD38A356E18A04" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_5D5E26F77EEAC39FFD2B38A356E2D447" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3B3E4E4C503D9CA450BD38A356E18A04" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_336D62827DE4C50E343C38A356E1FC44" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3B3E4E4C503D9CA450BD38A356E18A04" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_336D62827DE4C50E343C38A356E1FC44" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8EA6F7EC1FB455A4F58938A356E15C7C_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_336D62827DE4C50E343C38A356E1FC44" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8EA6F7EC1FB455A4F58938A356E15C7C_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8EA6F7EC1FB455A4F58938A356E15C7C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_336D62827DE4C50E343C38A356E1FC44" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8EA6F7EC1FB455A4F58938A356E15C7C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EisaiMember" xlink:label="loc_biib_EisaiMember_0D2966C5C8A73F89A58F38A356E111A8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_8EA6F7EC1FB455A4F58938A356E15C7C" xlink:to="loc_biib_EisaiMember_0D2966C5C8A73F89A58F38A356E111A8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_5F67D6A61CD46EB7F55138A356E2C76B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_3B3E4E4C503D9CA450BD38A356E18A04" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_5F67D6A61CD46EB7F55138A356E2C76B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_138DB7B5C6001BC6FD5D38A356E2F9E9_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_5F67D6A61CD46EB7F55138A356E2C76B" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_138DB7B5C6001BC6FD5D38A356E2F9E9_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_138DB7B5C6001BC6FD5D38A356E2F9E9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_5F67D6A61CD46EB7F55138A356E2C76B" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_138DB7B5C6001BC6FD5D38A356E2F9E9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BuildingMember" xlink:label="loc_us-gaap_BuildingMember_C5ED73E78CACC366017438A356E2887E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_138DB7B5C6001BC6FD5D38A356E2F9E9" xlink:to="loc_us-gaap_BuildingMember_C5ED73E78CACC366017438A356E2887E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MachineryAndEquipmentMember" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_4BE491E57AE73834EEAE38A356E267AB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_138DB7B5C6001BC6FD5D38A356E2F9E9" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_4BE491E57AE73834EEAE38A356E267AB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LandMember" xlink:label="loc_us-gaap_LandMember_8BAE76D781F1A88E70A538A356E2A46E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_138DB7B5C6001BC6FD5D38A356E2F9E9" xlink:to="loc_us-gaap_LandMember_8BAE76D781F1A88E70A538A356E2A46E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentAdditions" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAdditions_F471BED0362C729C95BD38A356E2E24D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_5D5E26F77EEAC39FFD2B38A356E2D447" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAdditions_F471BED0362C729C95BD38A356E2E24D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/PropertyPlantAndEquipmentTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/QuarterlyFinancialDataUnaudited" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/QuarterlyFinancialDataUnauditedDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/QuarterlyFinancialDataUnauditedDetailsTextual" xlink:type="extended">
    <loc xlink:href="biib-20151231.xsd#biib_ScheduleOfDevelopmentMilestoneAndCollaborationLineItems" xlink:label="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationLineItems_7CDF5BEB9B3A677577A893582D8F5DA4" xlink:type="locator" />
    <loc xlink:href="biib-20151231.xsd#biib_ScheduleOfDevelopmentMilestoneAndCollaborationTable" xlink:label="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationTable_7562D6F7CE86A7E34D2293582D8EF61F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationLineItems_7CDF5BEB9B3A677577A893582D8F5DA4" xlink:to="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationTable_7562D6F7CE86A7E34D2293582D8EF61F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_C9BCCA5614887BE79AEA93582D8E9ABC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationTable_7562D6F7CE86A7E34D2293582D8EF61F" xlink:to="loc_us-gaap_StatementGeographicalAxis_C9BCCA5614887BE79AEA93582D8E9ABC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_6D719E95E1A6D8BA33F893582D8EBA99_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_C9BCCA5614887BE79AEA93582D8E9ABC" xlink:to="loc_us-gaap_SegmentGeographicalDomain_6D719E95E1A6D8BA33F893582D8EBA99_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_6D719E95E1A6D8BA33F893582D8EBA99" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_C9BCCA5614887BE79AEA93582D8E9ABC" xlink:to="loc_us-gaap_SegmentGeographicalDomain_6D719E95E1A6D8BA33F893582D8EBA99" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_IT" xlink:label="loc_country_IT_EBB14A246D001B1F915493582D8EDE6D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_6D719E95E1A6D8BA33F893582D8EBA99" xlink:to="loc_country_IT_EBB14A246D001B1F915493582D8EDE6D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis" xlink:label="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_E66BD1B2C4C2F5FD784893582D8E5CF1" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationTable_7562D6F7CE86A7E34D2293582D8EF61F" xlink:to="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_E66BD1B2C4C2F5FD784893582D8E5CF1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ClassificationOfVariableInterestEntityDomain" xlink:label="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_40EBD9636490F1323CEF93582D8EEB41_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_E66BD1B2C4C2F5FD784893582D8E5CF1" xlink:to="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_40EBD9636490F1323CEF93582D8EEB41_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ClassificationOfVariableInterestEntityDomain" xlink:label="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_40EBD9636490F1323CEF93582D8EEB41" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis_E66BD1B2C4C2F5FD784893582D8E5CF1" xlink:to="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_40EBD9636490F1323CEF93582D8EEB41" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NeurimmuneMember" xlink:label="loc_biib_NeurimmuneMember_CD6DD8CDA6E45A21F96C93582D8E1A8F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassificationOfVariableInterestEntityDomain_40EBD9636490F1323CEF93582D8EEB41" xlink:to="loc_biib_NeurimmuneMember_CD6DD8CDA6E45A21F96C93582D8E1A8F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_0291529BB1210B62A8B693582D8ECEC3" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationTable_7562D6F7CE86A7E34D2293582D8EF61F" xlink:to="loc_us-gaap_TypeOfArrangementAxis_0291529BB1210B62A8B693582D8ECEC3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_386DB671ECFC303CA27593582D8FF132_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_0291529BB1210B62A8B693582D8ECEC3" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_386DB671ECFC303CA27593582D8FF132_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_386DB671ECFC303CA27593582D8FF132" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_0291529BB1210B62A8B693582D8ECEC3" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_386DB671ECFC303CA27593582D8FF132" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AGTCMember" xlink:label="loc_biib_AGTCMember_B48193D647AD3D5A7D1593582D8FDE14" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_386DB671ECFC303CA27593582D8FF132" xlink:to="loc_biib_AGTCMember_B48193D647AD3D5A7D1593582D8FDE14" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MitsubishiTanabePharmaCorporationMember" xlink:label="loc_biib_MitsubishiTanabePharmaCorporationMember_7941860A9B32CF4299F093582D8F5B73" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_386DB671ECFC303CA27593582D8FF132" xlink:to="loc_biib_MitsubishiTanabePharmaCorporationMember_7941860A9B32CF4299F093582D8F5B73" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EisaiMember" xlink:label="loc_biib_EisaiMember_72D93DF27F2045653BB993582D8F3A17" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_386DB671ECFC303CA27593582D8FF132" xlink:to="loc_biib_EisaiMember_72D93DF27F2045653BB993582D8F3A17" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_0982A2008736AE2F497A93582D8F3304" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationLineItems_7CDF5BEB9B3A677577A893582D8F5DA4" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_0982A2008736AE2F497A93582D8F3304" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_93C44E2EF15B0D6F10A193582D8F5120" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationLineItems_7CDF5BEB9B3A677577A893582D8F5DA4" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_93C44E2EF15B0D6F10A193582D8F5120" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_4C6FDBBFDB4990C670C093582D8F8B57" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationLineItems_7CDF5BEB9B3A677577A893582D8F5DA4" xlink:to="loc_us-gaap_RestructuringCharges_4C6FDBBFDB4990C670C093582D8F8B57" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_7BD2FD44548E7769BF6393582D8F1D6F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ScheduleOfDevelopmentMilestoneAndCollaborationLineItems_7CDF5BEB9B3A677577A893582D8F5DA4" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_7BD2FD44548E7769BF6393582D8F1D6F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/QuarterlyFinancialDataUnauditedTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/RestructuringRestructuring" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/RestructuringRestructuringDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_D0D228F95C32351A1526A34B6AB86200" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_4793816C3B7775B0D6ECA34B6AB886B4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_D0D228F95C32351A1526A34B6AB86200" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_4793816C3B7775B0D6ECA34B6AB886B4" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_58A37B23AFBEB876F54BA34B6AB86DDF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_4793816C3B7775B0D6ECA34B6AB886B4" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_58A37B23AFBEB876F54BA34B6AB86DDF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_FC9038997F7E17DB835AA34B6AB86FBC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_58A37B23AFBEB876F54BA34B6AB86DDF" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_FC9038997F7E17DB835AA34B6AB86FBC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_FC9038997F7E17DB835AA34B6AB86FBC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_58A37B23AFBEB876F54BA34B6AB86DDF" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_FC9038997F7E17DB835AA34B6AB86FBC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeSeveranceMember" xlink:label="loc_us-gaap_EmployeeSeveranceMember_6D6CEBFF9E17820C928EA34B6AB8677A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_FC9038997F7E17DB835AA34B6AB86FBC" xlink:to="loc_us-gaap_EmployeeSeveranceMember_6D6CEBFF9E17820C928EA34B6AB8677A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ContractTerminationMember" xlink:label="loc_us-gaap_ContractTerminationMember_AACDEB13A404D3499DD1A34B6AB86691" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_FC9038997F7E17DB835AA34B6AB86FBC" xlink:to="loc_us-gaap_ContractTerminationMember_AACDEB13A404D3499DD1A34B6AB86691" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_403A5AC5D36027E1FC56A34B6AB82CE9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_D0D228F95C32351A1526A34B6AB86200" xlink:to="loc_us-gaap_RestructuringCharges_403A5AC5D36027E1FC56A34B6AB82CE9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserveAccrualAdjustment" xlink:label="loc_us-gaap_RestructuringReserveAccrualAdjustment_8D70FC9AFF553E31F148A34B6AB8E3A6" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_D0D228F95C32351A1526A34B6AB86200" xlink:to="loc_us-gaap_RestructuringReserveAccrualAdjustment_8D70FC9AFF553E31F148A34B6AB8E3A6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsForRestructuring" xlink:label="loc_us-gaap_PaymentsForRestructuring_24842824D18CF28086C3A34B6AB90DFB" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_D0D228F95C32351A1526A34B6AB86200" xlink:to="loc_us-gaap_PaymentsForRestructuring_24842824D18CF28086C3A34B6AB90DFB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserve" xlink:label="loc_us-gaap_RestructuringReserve_83ADDA779D2D596CCC80A34B6AB92AD9" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_D0D228F95C32351A1526A34B6AB86200" xlink:to="loc_us-gaap_RestructuringReserve_83ADDA779D2D596CCC80A34B6AB92AD9" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/RestructuringRestructuringDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_100D276A9E1606774F4293582F46EAF2" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_9BE33509E755ABE8F0BA93582F4699DE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_100D276A9E1606774F4293582F46EAF2" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_9BE33509E755ABE8F0BA93582F4699DE" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_609773673EE18F66158593582F462080" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_9BE33509E755ABE8F0BA93582F4699DE" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_609773673EE18F66158593582F462080" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_1E36BCB41A357200179593582F46BFF8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_609773673EE18F66158593582F462080" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_1E36BCB41A357200179593582F46BFF8_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_1E36BCB41A357200179593582F46BFF8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_609773673EE18F66158593582F462080" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_1E36BCB41A357200179593582F46BFF8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeSeveranceMember" xlink:label="loc_us-gaap_EmployeeSeveranceMember_D2972B22204C90579FFA93582F46BD6A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_1E36BCB41A357200179593582F46BFF8" xlink:to="loc_us-gaap_EmployeeSeveranceMember_D2972B22204C90579FFA93582F46BD6A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ContractTerminationMember" xlink:label="loc_us-gaap_ContractTerminationMember_6FCAACD5BC492B1364FE93582F46FE6C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_1E36BCB41A357200179593582F46BFF8" xlink:to="loc_us-gaap_ContractTerminationMember_6FCAACD5BC492B1364FE93582F46FE6C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_Restructuringandrelatedcostexpectedpercentageofpositionseliminated" xlink:label="loc_biib_Restructuringandrelatedcostexpectedpercentageofpositionseliminated_36D6F8AC4923455F7EC993582F461A06" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_100D276A9E1606774F4293582F46EAF2" xlink:to="loc_biib_Restructuringandrelatedcostexpectedpercentageofpositionseliminated_36D6F8AC4923455F7EC993582F461A06" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RestructuringandRelatedCostExpectedCashPayments" xlink:label="loc_biib_RestructuringandRelatedCostExpectedCashPayments_4575AF495F5D1800146C93582F466615" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_100D276A9E1606774F4293582F46EAF2" xlink:to="loc_biib_RestructuringandRelatedCostExpectedCashPayments_4575AF495F5D1800146C93582F466615" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserveAccrualAdjustment" xlink:label="loc_us-gaap_RestructuringReserveAccrualAdjustment_5997A996DD1B835C326993582F467471" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_100D276A9E1606774F4293582F46EAF2" xlink:to="loc_us-gaap_RestructuringReserveAccrualAdjustment_5997A996DD1B835C326993582F467471" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringAndRelatedCostExpectedCost1" xlink:label="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_F342CD5793A41DDC243393582F478DA5" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_100D276A9E1606774F4293582F46EAF2" xlink:to="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_F342CD5793A41DDC243393582F478DA5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_EFE58B1659EFBA5A79CB93582F4755B0" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_100D276A9E1606774F4293582F46EAF2" xlink:to="loc_us-gaap_RestructuringCharges_EFE58B1659EFBA5A79CB93582F4755B0" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/RestructuringRestructuringTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/RevenueReserves" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/RevenueReservesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ValuationAndQualifyingAccountsDisclosureLineItems" xlink:label="loc_us-gaap_ValuationAndQualifyingAccountsDisclosureLineItems_54B09DAD77CB1B68BDCC38A353FA8B23" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ValuationAndQualifyingAccountsDisclosureTable" xlink:label="loc_us-gaap_ValuationAndQualifyingAccountsDisclosureTable_4A82E3FAF26B9C2426AA38A353FA2915" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ValuationAndQualifyingAccountsDisclosureLineItems_54B09DAD77CB1B68BDCC38A353FA8B23" xlink:to="loc_us-gaap_ValuationAndQualifyingAccountsDisclosureTable_4A82E3FAF26B9C2426AA38A353FA2915" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ValuationAllowancesAndReservesTypeAxis" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_D63BD7385AC0963A5CF438A353FA98D9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ValuationAndQualifyingAccountsDisclosureTable_4A82E3FAF26B9C2426AA38A353FA2915" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_D63BD7385AC0963A5CF438A353FA98D9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ValuationAllowancesAndReservesDomain" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_391120911795C250B1AB38A353FA9369_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_D63BD7385AC0963A5CF438A353FA98D9" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_391120911795C250B1AB38A353FA9369_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ValuationAllowancesAndReservesDomain" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesDomain_391120911795C250B1AB38A353FA9369" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesTypeAxis_D63BD7385AC0963A5CF438A353FA98D9" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesDomain_391120911795C250B1AB38A353FA9369" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReserveForCashDiscountMember" xlink:label="loc_us-gaap_ReserveForCashDiscountMember_9B39ED37AD49D9E3576E38A353FA262F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_391120911795C250B1AB38A353FA9369" xlink:to="loc_us-gaap_ReserveForCashDiscountMember_9B39ED37AD49D9E3576E38A353FA262F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ContractualAdjustmentsMember" xlink:label="loc_biib_ContractualAdjustmentsMember_2E81D210CC743515AB0738A353FA56E9" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_391120911795C250B1AB38A353FA9369" xlink:to="loc_biib_ContractualAdjustmentsMember_2E81D210CC743515AB0738A353FA56E9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesReturnsAndAllowancesMember" xlink:label="loc_us-gaap_SalesReturnsAndAllowancesMember_01521B5C408E1FB9D9AE38A353FA74BE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAllowancesAndReservesDomain_391120911795C250B1AB38A353FA9369" xlink:to="loc_us-gaap_SalesReturnsAndAllowancesMember_01521B5C408E1FB9D9AE38A353FA74BE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesAbstract" xlink:label="loc_biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesAbstract_875F345D9EAA1C17E2DB38A353FA7C1F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationAndQualifyingAccountsDisclosureLineItems_54B09DAD77CB1B68BDCC38A353FA8B23" xlink:to="loc_biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesAbstract_875F345D9EAA1C17E2DB38A353FA7C1F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ValuationAllowancesAndReservesBalance" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesBalance_A147B12175580DE2D9E338A353FB4288" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesAbstract_875F345D9EAA1C17E2DB38A353FA7C1F" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesBalance_A147B12175580DE2D9E338A353FB4288" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear" xlink:label="loc_biib_CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear_598CD8A6A3AF9491FA9C38A353FBF800" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesAbstract_875F345D9EAA1C17E2DB38A353FA7C1F" xlink:to="loc_biib_CurrentProvisionsToProductRevenueReservesRelatingToSalesInCurrentYear_598CD8A6A3AF9491FA9C38A353FBF800" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears" xlink:label="loc_biib_AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears_7D00426613B4B1CD174538A353FB7C9F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesAbstract_875F345D9EAA1C17E2DB38A353FA7C1F" xlink:to="loc_biib_AdjustmentsToProductRevenueReservesRelatingToSalesInPriorYears_7D00426613B4B1CD174538A353FB7C9F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear" xlink:label="loc_biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear_3E28DF57E223BD46E27A38A353FBAEB4" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesAbstract_875F345D9EAA1C17E2DB38A353FA7C1F" xlink:to="loc_biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToCurrentYear_3E28DF57E223BD46E27A38A353FBAEB4" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear" xlink:label="loc_biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear_28A04DE65941BC15A7F638A353FB4373" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_AnalysisOfAmountOfAndChangeInProductRevenueReservesAbstract_875F345D9EAA1C17E2DB38A353FA7C1F" xlink:to="loc_biib_PaymentsReturnsAppliedAgainstProductRevenueReservesRelatingToPriorYear_28A04DE65941BC15A7F638A353FB4373" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/RevenueReservesDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_F6554C06906DA44A828038A35231C877" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_25EC4D3ED1308FF50F2A38A352302AA4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_F6554C06906DA44A828038A35231C877" xlink:to="loc_us-gaap_StatementTable_25EC4D3ED1308FF50F2A38A352302AA4" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_FB22A40FE463D17A446C38A352309989" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_25EC4D3ED1308FF50F2A38A352302AA4" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_FB22A40FE463D17A446C38A352309989" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_7D5F37C75F539828EA6E38A3523078A3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_FB22A40FE463D17A446C38A352309989" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_7D5F37C75F539828EA6E38A3523078A3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_7D5F37C75F539828EA6E38A3523078A3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_FB22A40FE463D17A446C38A352309989" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_7D5F37C75F539828EA6E38A3523078A3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsReceivableMember" xlink:label="loc_us-gaap_AccountsReceivableMember_E579D86953A3E58406EC38A352318966" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_7D5F37C75F539828EA6E38A3523078A3" xlink:to="loc_us-gaap_AccountsReceivableMember_E579D86953A3E58406EC38A352318966" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCurrentLiabilitiesMember" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_6AEE8694837B9CC8E91A38A352317981" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_7D5F37C75F539828EA6E38A3523078A3" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_6AEE8694837B9CC8E91A38A352317981" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ValuationAllowancesAndReservesBalance" xlink:label="loc_us-gaap_ValuationAllowancesAndReservesBalance_41E31A5F12D66E79D11038A35231E128" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_F6554C06906DA44A828038A35231C877" xlink:to="loc_us-gaap_ValuationAllowancesAndReservesBalance_41E31A5F12D66E79D11038A35231E128" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/RevenueReservesTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/SegmentInformation" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/SegmentInformationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_1C649AE38FC47A6ED53B93582DB1C1D7" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_69288E60D2271F348B9B93582DB12F54" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_1C649AE38FC47A6ED53B93582DB1C1D7" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_69288E60D2271F348B9B93582DB12F54" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_E4BD828DD45D0414430593582DB10077" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_69288E60D2271F348B9B93582DB12F54" xlink:to="loc_us-gaap_ProductOrServiceAxis_E4BD828DD45D0414430593582DB10077" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_E4BD828DD45D0414430593582DB10077" xlink:to="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_E4BD828DD45D0414430593582DB10077" xlink:to="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TecfideraMember" xlink:label="loc_biib_TecfideraMember_AA323686622737C15D7993582DB1F6EB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:to="loc_biib_TecfideraMember_AA323686622737C15D7993582DB1F6EB" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AVONEXMember" xlink:label="loc_biib_AVONEXMember_51A775E1F2D69C848B0293BDFED76FA3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:to="loc_biib_AVONEXMember_51A775E1F2D69C848B0293BDFED76FA3" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PLEGRIDYMember" xlink:label="loc_biib_PLEGRIDYMember_4FF1B60F70EA75C024E593BF1760193D" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:to="loc_biib_PLEGRIDYMember_4FF1B60F70EA75C024E593BF1760193D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TysabriMember" xlink:label="loc_biib_TysabriMember_9E74EB0A8F1143A5102693582DB111AA" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:to="loc_biib_TysabriMember_9E74EB0A8F1143A5102693582DB111AA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FAMPYRAMember" xlink:label="loc_biib_FAMPYRAMember_E4159D68FB4AE8AA37FE93582DB17627" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:to="loc_biib_FAMPYRAMember_E4159D68FB4AE8AA37FE93582DB17627" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ELOCTATEMember" xlink:label="loc_biib_ELOCTATEMember_C939844255E3DA7EA53C93582DB1250A" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:to="loc_biib_ELOCTATEMember_C939844255E3DA7EA53C93582DB1250A" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ALPROLIXProductMember" xlink:label="loc_biib_ALPROLIXProductMember_4B031756E9332D9CBFA993582DB16CB2" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:to="loc_biib_ALPROLIXProductMember_4B031756E9332D9CBFA993582DB16CB2" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_FUMADERMMember" xlink:label="loc_biib_FUMADERMMember_2644BCB933B4CD111AF393582DB100DE" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_385D02028F6774F8A9DA93582DB18F63" xlink:to="loc_biib_FUMADERMMember_2644BCB933B4CD111AF393582DB100DE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_0D8B1A8B504FEB2113BC93582DB1F72D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_69288E60D2271F348B9B93582DB12F54" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_0D8B1A8B504FEB2113BC93582DB1F72D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_FA29F4DA416A7E8A095B93582DB11FFC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_0D8B1A8B504FEB2113BC93582DB1F72D" xlink:to="loc_us-gaap_SegmentDomain_FA29F4DA416A7E8A095B93582DB11FFC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_FA29F4DA416A7E8A095B93582DB11FFC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_0D8B1A8B504FEB2113BC93582DB1F72D" xlink:to="loc_us-gaap_SegmentDomain_FA29F4DA416A7E8A095B93582DB11FFC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_US" xlink:label="loc_country_US_ED064019BF23D178BFFB93582DB1DA08" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_FA29F4DA416A7E8A095B93582DB11FFC" xlink:to="loc_country_US_ED064019BF23D178BFFB93582DB1DA08" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RestOfWorldMember" xlink:label="loc_biib_RestOfWorldMember_6104F7C6E215BA8B5F8593582DB1F25C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_FA29F4DA416A7E8A095B93582DB11FFC" xlink:to="loc_biib_RestOfWorldMember_6104F7C6E215BA8B5F8593582DB1F25C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNetAbstract" xlink:label="loc_us-gaap_SalesRevenueGoodsNetAbstract_F0F14FB3439325F9222893582DB271E0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_1C649AE38FC47A6ED53B93582DB1C1D7" xlink:to="loc_us-gaap_SalesRevenueGoodsNetAbstract_F0F14FB3439325F9222893582DB271E0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_FC3BACCB842A6E5D605193582DB28ADD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SalesRevenueGoodsNetAbstract_F0F14FB3439325F9222893582DB271E0" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_FC3BACCB842A6E5D605193582DB28ADD" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/SegmentInformationDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:label="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_AAC54936E48FB1EA8CDC38A3526AEAA8" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:label="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_41189C1D1B76EC0A46E438A3526A6553" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_AAC54936E48FB1EA8CDC38A3526AEAA8" xlink:to="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_41189C1D1B76EC0A46E438A3526A6553" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_311969CF7FB375C66B5238A3526A3C62" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_41189C1D1B76EC0A46E438A3526A6553" xlink:to="loc_us-gaap_StatementGeographicalAxis_311969CF7FB375C66B5238A3526A3C62" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_311969CF7FB375C66B5238A3526A3C62" xlink:to="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_311969CF7FB375C66B5238A3526A3C62" xlink:to="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_US" xlink:label="loc_country_US_583278D6AB2DE841CEFF38A3526A41C7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB" xlink:to="loc_country_US_583278D6AB2DE841CEFF38A3526A41C7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EuropeMember" xlink:label="loc_us-gaap_EuropeMember_22DA1AF86DCCC1149D0438A3526A5D05" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB" xlink:to="loc_us-gaap_EuropeMember_22DA1AF86DCCC1149D0438A3526A5D05" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_DE" xlink:label="loc_country_DE_BB7ABBF54FFCA885765238A3526ABF89" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB" xlink:to="loc_country_DE_BB7ABBF54FFCA885765238A3526ABF89" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AsiaMember" xlink:label="loc_us-gaap_AsiaMember_37EDA8542BEF8885EA7C38A3526A278D" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB" xlink:to="loc_us-gaap_AsiaMember_37EDA8542BEF8885EA7C38A3526A278D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SegmentGeographicalGroupsOfCountriesGroupOtherMember" xlink:label="loc_biib_SegmentGeographicalGroupsOfCountriesGroupOtherMember_78A5D0860C9F0B41068D38A3526AE776" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_38C3F2E633AC5085F23038A3526A21BB" xlink:to="loc_biib_SegmentGeographicalGroupsOfCountriesGroupOtherMember_78A5D0860C9F0B41068D38A3526AE776" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentsGeographicalAreasAbstract" xlink:label="loc_us-gaap_SegmentsGeographicalAreasAbstract_354E0164C1BB8CC0F85638A3526AD552" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_AAC54936E48FB1EA8CDC38A3526AEAA8" xlink:to="loc_us-gaap_SegmentsGeographicalAreasAbstract_354E0164C1BB8CC0F85638A3526AD552" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="loc_us-gaap_SalesRevenueGoodsNet_13E3187695FAB6D75BC538A3526ABA23" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentsGeographicalAreasAbstract_354E0164C1BB8CC0F85638A3526AD552" xlink:to="loc_us-gaap_SalesRevenueGoodsNet_13E3187695FAB6D75BC538A3526ABA23" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RevenuesFromUnconsolidatedJointBusiness" xlink:label="loc_biib_RevenuesFromUnconsolidatedJointBusiness_F2EB0BEC9C01139352A838A3526BB69A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentsGeographicalAreasAbstract_354E0164C1BB8CC0F85638A3526AD552" xlink:to="loc_biib_RevenuesFromUnconsolidatedJointBusiness_F2EB0BEC9C01139352A838A3526BB69A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherSalesRevenueNet" xlink:label="loc_us-gaap_OtherSalesRevenueNet_2082A334E47ECC84D18A38A3526B420A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentsGeographicalAreasAbstract_354E0164C1BB8CC0F85638A3526AD552" xlink:to="loc_us-gaap_OtherSalesRevenueNet_2082A334E47ECC84D18A38A3526B420A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncurrentAssets" xlink:label="loc_us-gaap_NoncurrentAssets_4270173758342207655B38A3526B85F2" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentsGeographicalAreasAbstract_354E0164C1BB8CC0F85638A3526AD552" xlink:to="loc_us-gaap_NoncurrentAssets_4270173758342207655B38A3526B85F2" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/SegmentInformationDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_EFCF28BE687BE3AEA68F38A3565ADC02" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_F3A7F53566CAABF32CC338A3565ABC80" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_EFCF28BE687BE3AEA68F38A3565ADC02" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_F3A7F53566CAABF32CC338A3565ABC80" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_E956AC05C35633591A1438A3565AC747" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_F3A7F53566CAABF32CC338A3565ABC80" xlink:to="loc_us-gaap_StatementGeographicalAxis_E956AC05C35633591A1438A3565AC747" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_81706E1B9664BF36C56838A3565A2D15_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_E956AC05C35633591A1438A3565AC747" xlink:to="loc_us-gaap_SegmentGeographicalDomain_81706E1B9664BF36C56838A3565A2D15_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_81706E1B9664BF36C56838A3565A2D15" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_E956AC05C35633591A1438A3565AC747" xlink:to="loc_us-gaap_SegmentGeographicalDomain_81706E1B9664BF36C56838A3565A2D15" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_DK" xlink:label="loc_country_DK_5E7BC962BE53087A9B5A38A3565AA832" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_81706E1B9664BF36C56838A3565A2D15" xlink:to="loc_country_DK_5E7BC962BE53087A9B5A38A3565AA832" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MajorDistributorAxis" xlink:label="loc_biib_MajorDistributorAxis_00507505375E9ADAC02E38A3565A9BAF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_F3A7F53566CAABF32CC338A3565ABC80" xlink:to="loc_biib_MajorDistributorAxis_00507505375E9ADAC02E38A3565A9BAF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MajorDistributorDomain" xlink:label="loc_biib_MajorDistributorDomain_9A07EE8DA25FE78A384738A3565A5067_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_MajorDistributorAxis_00507505375E9ADAC02E38A3565A9BAF" xlink:to="loc_biib_MajorDistributorDomain_9A07EE8DA25FE78A384738A3565A5067_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MajorDistributorDomain" xlink:label="loc_biib_MajorDistributorDomain_9A07EE8DA25FE78A384738A3565A5067" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_MajorDistributorAxis_00507505375E9ADAC02E38A3565A9BAF" xlink:to="loc_biib_MajorDistributorDomain_9A07EE8DA25FE78A384738A3565A5067" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DistributorOneMember" xlink:label="loc_biib_DistributorOneMember_5AD69CCFD0D67FE4E3DF38A3565A54FD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MajorDistributorDomain_9A07EE8DA25FE78A384738A3565A5067" xlink:to="loc_biib_DistributorOneMember_5AD69CCFD0D67FE4E3DF38A3565A54FD" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DistributorTwoMember" xlink:label="loc_biib_DistributorTwoMember_EF1B70F2C2B319385F2838A3565A0282" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MajorDistributorDomain_9A07EE8DA25FE78A384738A3565A5067" xlink:to="loc_biib_DistributorTwoMember_EF1B70F2C2B319385F2838A3565A0282" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RevenuesInPercentageFromUnconsolidatedJointBusiness" xlink:label="loc_biib_RevenuesInPercentageFromUnconsolidatedJointBusiness_55E7AC59066BF8A147BC38A3565A0207" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_EFCF28BE687BE3AEA68F38A3565ADC02" xlink:to="loc_biib_RevenuesInPercentageFromUnconsolidatedJointBusiness_55E7AC59066BF8A147BC38A3565A0207" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EntityWidePercentageOfRevenueFromMajorDistributors" xlink:label="loc_biib_EntityWidePercentageOfRevenueFromMajorDistributors_D5EC9ED7E6822D2ABBF238A3565A66E0" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_EFCF28BE687BE3AEA68F38A3565ADC02" xlink:to="loc_biib_EntityWidePercentageOfRevenueFromMajorDistributors_D5EC9ED7E6822D2ABBF238A3565A66E0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NoncurrentAssets" xlink:label="loc_us-gaap_NoncurrentAssets_33051466D157CF1100BE38A3565A596F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_EFCF28BE687BE3AEA68F38A3565ADC02" xlink:to="loc_us-gaap_NoncurrentAssets_33051466D157CF1100BE38A3565A596F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/SegmentInformationTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPayments" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_CB93F131642F96A37C8C93D5C9F6BBAD" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_A3200CB417915E39AC4193D5C9F54B54" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_CB93F131642F96A37C8C93D5C9F6BBAD" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_A3200CB417915E39AC4193D5C9F54B54" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_8D19CD154CC2BC16542C93D5C9F594D6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_A3200CB417915E39AC4193D5C9F54B54" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_8D19CD154CC2BC16542C93D5C9F594D6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_1631EB288E4E0F0D45BB93D5C9F585CC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_8D19CD154CC2BC16542C93D5C9F594D6" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_1631EB288E4E0F0D45BB93D5C9F585CC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_1631EB288E4E0F0D45BB93D5C9F585CC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_8D19CD154CC2BC16542C93D5C9F594D6" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_1631EB288E4E0F0D45BB93D5C9F585CC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_83FBAD2106C8946A03CF93D5C9F69F16" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1631EB288E4E0F0D45BB93D5C9F585CC" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_83FBAD2106C8946A03CF93D5C9F69F16" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_4A50D7796B3A3D60369A93D5C9F65EB1" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1631EB288E4E0F0D45BB93D5C9F585CC" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_4A50D7796B3A3D60369A93D5C9F65EB1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringChargesMember" xlink:label="loc_us-gaap_RestructuringChargesMember_0EC24D24F01A41296D6393D5C9F6F696" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1631EB288E4E0F0D45BB93D5C9F585CC" xlink:to="loc_us-gaap_RestructuringChargesMember_0EC24D24F01A41296D6393D5C9F6F696" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ParentMember" xlink:label="loc_us-gaap_ParentMember_D31ED573CC1379AE2E6793D5C9F6B956" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_1631EB288E4E0F0D45BB93D5C9F585CC" xlink:to="loc_us-gaap_ParentMember_D31ED573CC1379AE2E6793D5C9F6B956" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_3DB9C423B012BFA135C293D5C9F61DA3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_CB93F131642F96A37C8C93D5C9F6BBAD" xlink:to="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_3DB9C423B012BFA135C293D5C9F61DA3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_E7FED733DC3A15639D6293D5C9F61454" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_3DB9C423B012BFA135C293D5C9F61DA3" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_E7FED733DC3A15639D6293D5C9F61454" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_4FF3132759B6BE9EDA1C93D5C9F60A6E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_3DB9C423B012BFA135C293D5C9F61DA3" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_4FF3132759B6BE9EDA1C93D5C9F60A6E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_A21069CFFFA1F1609C8C93D5C9F6B956" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_3DB9C423B012BFA135C293D5C9F61DA3" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_A21069CFFFA1F1609C8C93D5C9F6B956" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses" xlink:label="loc_biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses_5D606F6D385779F6C91F93D5C9F6313F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_3DB9C423B012BFA135C293D5C9F61DA3" xlink:to="loc_biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses_5D606F6D385779F6C91F93D5C9F6313F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_17E570DCFB9764669A0893D5C9F6209F" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationAllocationAndClassificationInFinancialStatementsAbstract_3DB9C423B012BFA135C293D5C9F61DA3" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_17E570DCFB9764669A0893D5C9F6209F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails1" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DDBA53D56B3C9E0B722E38A351C42815" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_A1260F14EA75B6FC484738A351C3902A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DDBA53D56B3C9E0B722E38A351C42815" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_A1260F14EA75B6FC484738A351C3902A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_D62D6EECD38B98DB398D38A351C33F43" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_A1260F14EA75B6FC484738A351C3902A" xlink:to="loc_us-gaap_PlanNameAxis_D62D6EECD38B98DB398D38A351C33F43" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_D62D6EECD38B98DB398D38A351C33F43" xlink:to="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_D62D6EECD38B98DB398D38A351C33F43" xlink:to="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_5FB678B51102E88068F138A351C43F1A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A" xlink:to="loc_us-gaap_EmployeeStockOptionMember_5FB678B51102E88068F138A351C43F1A" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MarketStockUnitsMember" xlink:label="loc_biib_MarketStockUnitsMember_47C8FB953AAFDF7742FB38A351C4910E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A" xlink:to="loc_biib_MarketStockUnitsMember_47C8FB953AAFDF7742FB38A351C4910E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TimeVestedRestrictedStockUnitsMember" xlink:label="loc_biib_TimeVestedRestrictedStockUnitsMember_60E4AEE4672C29A51E9B38A351C4099C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A" xlink:to="loc_biib_TimeVestedRestrictedStockUnitsMember_60E4AEE4672C29A51E9B38A351C4099C" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CashSettledPerformanceSharesMember" xlink:label="loc_biib_CashSettledPerformanceSharesMember_9B089081790B1CE751F338A351C4E383" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A" xlink:to="loc_biib_CashSettledPerformanceSharesMember_9B089081790B1CE751F338A351C4E383" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PerformanceSharesMember" xlink:label="loc_us-gaap_PerformanceSharesMember_7D8B76DBC0E4EAFCF18638A351C49BAF" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A" xlink:to="loc_us-gaap_PerformanceSharesMember_7D8B76DBC0E4EAFCF18638A351C49BAF" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EmployeeStockPurchasePlanMember" xlink:label="loc_biib_EmployeeStockPurchasePlanMember_3708315C2599A53C59E338A351C41D8D" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4EC1B390895D7D88578138A351C40C0A" xlink:to="loc_biib_EmployeeStockPurchasePlanMember_3708315C2599A53C59E338A351C41D8D" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract" xlink:label="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_33D34398B8C811D73C5738A351C48C32" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_DDBA53D56B3C9E0B722E38A351C42815" xlink:to="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_33D34398B8C811D73C5738A351C48C32" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_7223C27190003AE1DAE138A351C40567" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_33D34398B8C811D73C5738A351C48C32" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_7223C27190003AE1DAE138A351C40567" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_166B2321EEC6FA7D93A638A351C445C7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_33D34398B8C811D73C5738A351C48C32" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_166B2321EEC6FA7D93A638A351C445C7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_B75367DC93997B4D31F438A351C47ED8" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_33D34398B8C811D73C5738A351C48C32" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_B75367DC93997B4D31F438A351C47ED8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses" xlink:label="loc_biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses_1541A7E48B0D186FA5EA38A351C42A33" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_SummaryOfShareBasedCompensationExpenseAssociatedWithDifferentProgramsAbstract_33D34398B8C811D73C5738A351C48C32" xlink:to="loc_biib_ShareBasedCompensationExpenseIncludedInCostsAndExpenses_1541A7E48B0D186FA5EA38A351C42A33" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails2" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails3" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails4" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_B56BCEE5AC256ABAA97E38A3534B9F13" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1BE9ECD4F99F22768B6F38A3534AF239" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_B56BCEE5AC256ABAA97E38A3534B9F13" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1BE9ECD4F99F22768B6F38A3534AF239" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_5734004D80954E4494DB38A3534B51B9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1BE9ECD4F99F22768B6F38A3534AF239" xlink:to="loc_us-gaap_AwardTypeAxis_5734004D80954E4494DB38A3534B51B9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5FA16734B2850DF100E938A3534BF2C1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_5734004D80954E4494DB38A3534B51B9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5FA16734B2850DF100E938A3534BF2C1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5FA16734B2850DF100E938A3534BF2C1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_5734004D80954E4494DB38A3534B51B9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5FA16734B2850DF100E938A3534BF2C1" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MarketStockUnitsMember" xlink:label="loc_biib_MarketStockUnitsMember_F7AA432E10402D1CC16738A3534B8D5F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5FA16734B2850DF100E938A3534BF2C1" xlink:to="loc_biib_MarketStockUnitsMember_F7AA432E10402D1CC16738A3534B8D5F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MarketStockUnitsActivityAbstract" xlink:label="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_B56BCEE5AC256ABAA97E38A3534B9F13" xlink:to="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_42665CAE190FBEDAE6D538A3534BA65F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_42665CAE190FBEDAE6D538A3534BA65F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_55F6DF417BE4B009718238A3534BA6FB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_55F6DF417BE4B009718238A3534BA6FB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_238769352ABA1FBB41E338A3534BBCC6" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_238769352ABA1FBB41E338A3534BBCC6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_659CF0CEEDAE07E529F038A3534BA9A8" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_659CF0CEEDAE07E529F038A3534BA9A8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_5E13630B2D751574DA8B38A3534B9FF8" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_5E13630B2D751574DA8B38A3534B9FF8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_9868E37B9AF59CAA3AF538A3534B6E47" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_9868E37B9AF59CAA3AF538A3534B6E47" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_6ECF9E36B3754DF6EB7C38A3534BAFB9" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_6ECF9E36B3754DF6EB7C38A3534BAFB9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_E0FE6EB810CF938AEC9F38A3534B3DA7" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MarketStockUnitsActivityAbstract_9CF96C04B9EBB2BF39A838A3534B4AD6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_E0FE6EB810CF938AEC9F38A3534B3DA7" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails5" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5F92934CDBECD2F9C64C93D5C9E14F20" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6F072C14B4E6B655C7AE93D5C9E1A820" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5F92934CDBECD2F9C64C93D5C9E14F20" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6F072C14B4E6B655C7AE93D5C9E1A820" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_4646B8E800F75C5057C093D5C9E11548" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6F072C14B4E6B655C7AE93D5C9E1A820" xlink:to="loc_us-gaap_AwardTypeAxis_4646B8E800F75C5057C093D5C9E11548" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3E4E07DE9E60B90A723593D5C9E18AB3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_4646B8E800F75C5057C093D5C9E11548" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3E4E07DE9E60B90A723593D5C9E18AB3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3E4E07DE9E60B90A723593D5C9E18AB3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_4646B8E800F75C5057C093D5C9E11548" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3E4E07DE9E60B90A723593D5C9E18AB3" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MarketStockUnitsMember" xlink:label="loc_biib_MarketStockUnitsMember_A9C3B692A59FE7947DAD93D5C9E14B6F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3E4E07DE9E60B90A723593D5C9E18AB3" xlink:to="loc_biib_MarketStockUnitsMember_A9C3B692A59FE7947DAD93D5C9E14B6F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_0618DEEF8496D620823393D5C9E1931A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6F072C14B4E6B655C7AE93D5C9E1A820" xlink:to="loc_us-gaap_RangeAxis_0618DEEF8496D620823393D5C9E1931A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_F998FCEF3B2980031D9993D5C9E1BBE6_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_0618DEEF8496D620823393D5C9E1931A" xlink:to="loc_us-gaap_RangeMember_F998FCEF3B2980031D9993D5C9E1BBE6_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_F998FCEF3B2980031D9993D5C9E1BBE6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_0618DEEF8496D620823393D5C9E1931A" xlink:to="loc_us-gaap_RangeMember_F998FCEF3B2980031D9993D5C9E1BBE6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_E0820DB682165630461C93D5C9E116D7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_F998FCEF3B2980031D9993D5C9E1BBE6" xlink:to="loc_us-gaap_MinimumMember_E0820DB682165630461C93D5C9E116D7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_E3FD526013C6FD21D34793D5C9E19420" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_F998FCEF3B2980031D9993D5C9E1BBE6" xlink:to="loc_us-gaap_MaximumMember_E3FD526013C6FD21D34793D5C9E19420" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ValuationOfGrantedStockUnitsAbstract" xlink:label="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5F92934CDBECD2F9C64C93D5C9E14F20" xlink:to="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_98FA3713CB727EDBB9EE93D5C9E1707A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_98FA3713CB727EDBB9EE93D5C9E1707A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_C0B74D6A2907A7BEA38093D5C9E15A2A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_C0B74D6A2907A7BEA38093D5C9E15A2A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_9270EAF2B76108E47E8C93D5C9E10421" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_9270EAF2B76108E47E8C93D5C9E10421" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_8CE6724163294A70034E93D5C9E15626" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_8CE6724163294A70034E93D5C9E15626" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AverageStockPriceOnGrantDateInDollarsMinimum" xlink:label="loc_biib_AverageStockPriceOnGrantDateInDollarsMinimum_B5A4A55E29F678A7735293D5C9E1359A" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:to="loc_biib_AverageStockPriceOnGrantDateInDollarsMinimum_B5A4A55E29F678A7735293D5C9E1359A" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AverageStockPriceOnGrantDateInDollarsMaximum" xlink:label="loc_biib_AverageStockPriceOnGrantDateInDollarsMaximum_9F2EBAFAD48B0C56739193D5C9E17280" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:to="loc_biib_AverageStockPriceOnGrantDateInDollarsMaximum_9F2EBAFAD48B0C56739193D5C9E17280" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_2AA64739946C70DB114393D5C9E2EF6C" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ValuationOfGrantedStockUnitsAbstract_ADC40861B43B3A805F5293D5C9E134BD" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_2AA64739946C70DB114393D5C9E2EF6C" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails6" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E8652754D1C974B2E2AF38A3556C6FD0" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_413BFD79D095683FAD3E38A3556C5795" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E8652754D1C974B2E2AF38A3556C6FD0" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_413BFD79D095683FAD3E38A3556C5795" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_B8875B37C21458C1430438A3556C2AAF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_413BFD79D095683FAD3E38A3556C5795" xlink:to="loc_us-gaap_AwardTypeAxis_B8875B37C21458C1430438A3556C2AAF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0A07D03EA37D677884BF38A3556C6667_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_B8875B37C21458C1430438A3556C2AAF" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0A07D03EA37D677884BF38A3556C6667_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0A07D03EA37D677884BF38A3556C6667" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_B8875B37C21458C1430438A3556C2AAF" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0A07D03EA37D677884BF38A3556C6667" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CashSettledPerformanceSharesMember" xlink:label="loc_biib_CashSettledPerformanceSharesMember_C046B76FB400E76AC39E38A3556C91BA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0A07D03EA37D677884BF38A3556C6667" xlink:to="loc_biib_CashSettledPerformanceSharesMember_C046B76FB400E76AC39E38A3556C91BA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CashSettledPerformanceSharesAbstract" xlink:label="loc_biib_CashSettledPerformanceSharesAbstract_C34FCB5137F8B48C707E38A3556CDB50" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E8652754D1C974B2E2AF38A3556C6FD0" xlink:to="loc_biib_CashSettledPerformanceSharesAbstract_C34FCB5137F8B48C707E38A3556CDB50" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_64F6BE16BF74B554900338A3556D80C1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CashSettledPerformanceSharesAbstract_C34FCB5137F8B48C707E38A3556CDB50" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_64F6BE16BF74B554900338A3556D80C1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_17D4B97A6D6C18A618FD38A3556DFDFC" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CashSettledPerformanceSharesAbstract_C34FCB5137F8B48C707E38A3556CDB50" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_17D4B97A6D6C18A618FD38A3556DFDFC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_AD5E5843647D156230CE38A3556D13FA" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CashSettledPerformanceSharesAbstract_C34FCB5137F8B48C707E38A3556CDB50" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_AD5E5843647D156230CE38A3556D13FA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_6B5AAE4E80BE17FDE6DD38A3556D823E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_CashSettledPerformanceSharesAbstract_C34FCB5137F8B48C707E38A3556CDB50" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_6B5AAE4E80BE17FDE6DD38A3556D823E" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails8" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_A3CD57A145C20D0736C438A3524BE4F4" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1897B6E0300D4C20A7BB38A3524A9159" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_A3CD57A145C20D0736C438A3524BE4F4" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1897B6E0300D4C20A7BB38A3524A9159" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_59E0525AD706CEB260BD38A3524B7F42" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_1897B6E0300D4C20A7BB38A3524A9159" xlink:to="loc_us-gaap_AwardTypeAxis_59E0525AD706CEB260BD38A3524B7F42" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8C0A09213216E1AB6E7438A3524B468F_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_59E0525AD706CEB260BD38A3524B7F42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8C0A09213216E1AB6E7438A3524B468F_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8C0A09213216E1AB6E7438A3524B468F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_59E0525AD706CEB260BD38A3524B7F42" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8C0A09213216E1AB6E7438A3524B468F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TimeVestedRestrictedStockUnitsMember" xlink:label="loc_biib_TimeVestedRestrictedStockUnitsMember_4A05EAD367A6A271607B38A3524BFEF4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8C0A09213216E1AB6E7438A3524B468F" xlink:to="loc_biib_TimeVestedRestrictedStockUnitsMember_4A05EAD367A6A271607B38A3524BFEF4" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RestrictedStockUnitsActivityAbstract" xlink:label="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_A3CD57A145C20D0736C438A3524BE4F4" xlink:to="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_6D7B6A169ABDF8D8F7D038A3524B74CE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_6D7B6A169ABDF8D8F7D038A3524B74CE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_8161A1F63DB5C454795C38A3524B1BE2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_8161A1F63DB5C454795C38A3524B1BE2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_2BC356A55B151B1810F838A3524B28E1" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_2BC356A55B151B1810F838A3524B28E1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_8D47E8E0566C2AE700DE38A3524B62C2" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_8D47E8E0566C2AE700DE38A3524B62C2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_34C8CE9B59A99EF230E538A3524B445C" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_34C8CE9B59A99EF230E538A3524B445C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_B9A9B37B33E9939DECB238A3524B5F72" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_B9A9B37B33E9939DECB238A3524B5F72" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_3FE41D41256140F5B38F38A3524B6B25" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_3FE41D41256140F5B38F38A3524B6B25" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_80095C700D997D1936B038A3524B20A8" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_RestrictedStockUnitsActivityAbstract_5D250F23F9EC1873D96C38A3524B4587" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_80095C700D997D1936B038A3524B20A8" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetails9" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:label="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_F1F2DFF925DD78279ABB38A3553E6A73" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable_06DA34F5BA55FD26713C38A3553EE0D6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_F1F2DFF925DD78279ABB38A3553E6A73" xlink:to="loc_us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable_06DA34F5BA55FD26713C38A3553EE0D6" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_61F0B20E9C75EB6848DB38A3553E4689" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable_06DA34F5BA55FD26713C38A3553EE0D6" xlink:to="loc_us-gaap_PlanNameAxis_61F0B20E9C75EB6848DB38A3553E4689" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_057C8AD5FD561AA629E338A3553E4247_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_61F0B20E9C75EB6848DB38A3553E4689" xlink:to="loc_us-gaap_PlanNameDomain_057C8AD5FD561AA629E338A3553E4247_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_057C8AD5FD561AA629E338A3553E4247" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_61F0B20E9C75EB6848DB38A3553E4689" xlink:to="loc_us-gaap_PlanNameDomain_057C8AD5FD561AA629E338A3553E4247" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_A2015ESPPMember" xlink:label="loc_biib_A2015ESPPMember_2F37A9AA31DCE4F00E7238A3553E5983" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_057C8AD5FD561AA629E338A3553E4247" xlink:to="loc_biib_A2015ESPPMember_2F37A9AA31DCE4F00E7238A3553E5983" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_A1995ESPPMember" xlink:label="loc_biib_A1995ESPPMember_50181B306D5DD784FDB938A3553ED876" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_057C8AD5FD561AA629E338A3553E4247" xlink:to="loc_biib_A1995ESPPMember_50181B306D5DD784FDB938A3553ED876" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_29FD4852B22FC9A7A0DE38A3553E5ED0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_F1F2DFF925DD78279ABB38A3553E6A73" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_29FD4852B22FC9A7A0DE38A3553E5ED0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_E14C1F0C38DEF3E95CAD38A3553E388F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems_F1F2DFF925DD78279ABB38A3553E6A73" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_E14C1F0C38DEF3E95CAD38A3553E388F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsDetailsTextual" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_98E427C7314B6526C91B98B5C4EA7F3A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_98E427C7314B6526C91B98B5C4EA7F3A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingAxis" xlink:label="loc_us-gaap_VestingAxis_40E8C87B70D34F424B0198B5C4EBBA46" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_98E427C7314B6526C91B98B5C4EA7F3A" xlink:to="loc_us-gaap_VestingAxis_40E8C87B70D34F424B0198B5C4EBBA46" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingDomain" xlink:label="loc_us-gaap_VestingDomain_13260DB5A6AF6849214F98B5C4EBD1D1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_40E8C87B70D34F424B0198B5C4EBBA46" xlink:to="loc_us-gaap_VestingDomain_13260DB5A6AF6849214F98B5C4EBD1D1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingDomain" xlink:label="loc_us-gaap_VestingDomain_13260DB5A6AF6849214F98B5C4EBD1D1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_40E8C87B70D34F424B0198B5C4EBBA46" xlink:to="loc_us-gaap_VestingDomain_13260DB5A6AF6849214F98B5C4EBD1D1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_D42223038A32C35DD0E198B5C4EBBD43" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_13260DB5A6AF6849214F98B5C4EBD1D1" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_D42223038A32C35DD0E198B5C4EBBD43" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="loc_us-gaap_TitleOfIndividualAxis_41167DC81BCD8DB47F6998B5C4EC6B0D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_98E427C7314B6526C91B98B5C4EA7F3A" xlink:to="loc_us-gaap_TitleOfIndividualAxis_41167DC81BCD8DB47F6998B5C4EC6B0D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_DFE56EFFA4168C169F0698B5C4ECD2D6_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TitleOfIndividualAxis_41167DC81BCD8DB47F6998B5C4EC6B0D" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_DFE56EFFA4168C169F0698B5C4ECD2D6_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_DFE56EFFA4168C169F0698B5C4ECD2D6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TitleOfIndividualAxis_41167DC81BCD8DB47F6998B5C4EC6B0D" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_DFE56EFFA4168C169F0698B5C4ECD2D6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DirectorMember" xlink:label="loc_us-gaap_DirectorMember_BA32105F96DE220C366598B5C4ECA349" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_DFE56EFFA4168C169F0698B5C4ECD2D6" xlink:to="loc_us-gaap_DirectorMember_BA32105F96DE220C366598B5C4ECA349" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ShareBasedCompensationPlanAxis" xlink:label="loc_biib_ShareBasedCompensationPlanAxis_4063594AD7691594A39498B5C4EC7522" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_98E427C7314B6526C91B98B5C4EA7F3A" xlink:to="loc_biib_ShareBasedCompensationPlanAxis_4063594AD7691594A39498B5C4EC7522" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ShareBasedCompensationPlanDomain" xlink:label="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_ShareBasedCompensationPlanAxis_4063594AD7691594A39498B5C4EC7522" xlink:to="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_ShareBasedCompensationPlanDomain" xlink:label="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_ShareBasedCompensationPlanAxis_4063594AD7691594A39498B5C4EC7522" xlink:to="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DirectorsPlanMember" xlink:label="loc_biib_DirectorsPlanMember_A3EE5948A54486A74ECF98B5C4EC55DA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49" xlink:to="loc_biib_DirectorsPlanMember_A3EE5948A54486A74ECF98B5C4EC55DA" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_OmnibusPlansMember" xlink:label="loc_biib_OmnibusPlansMember_4DEFADDA318BD6DB40B098B5C4EDB1C7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49" xlink:to="loc_biib_OmnibusPlansMember_4DEFADDA318BD6DB40B098B5C4EDB1C7" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AttainmentOfPerformanceCriteriain2014Member" xlink:label="loc_biib_AttainmentOfPerformanceCriteriain2014Member_159C56D155D476E1AB1398B5C4ED79D9" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49" xlink:to="loc_biib_AttainmentOfPerformanceCriteriain2014Member_159C56D155D476E1AB1398B5C4ED79D9" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AttainmentOfPerformanceCriteriain2013Member" xlink:label="loc_biib_AttainmentOfPerformanceCriteriain2013Member_FD04B19BDA389B0E466398B5C4EDF891" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49" xlink:to="loc_biib_AttainmentOfPerformanceCriteriain2013Member_FD04B19BDA389B0E466398B5C4EDF891" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AttainmentOfPerformanceCriteriain2012Member" xlink:label="loc_biib_AttainmentOfPerformanceCriteriain2012Member_EDEA18D7F463F31C5E9B98B5C4EDACEC" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49" xlink:to="loc_biib_AttainmentOfPerformanceCriteriain2012Member_EDEA18D7F463F31C5E9B98B5C4EDACEC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_AttainmentOfPerformanceCriteriain2011Member" xlink:label="loc_biib_AttainmentOfPerformanceCriteriain2011Member_1ADE04A9800A9514912B98B5C4EDD51D" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_ShareBasedCompensationPlanDomain_BC0AED3A0F6D643197DE98B5C4EC3F49" xlink:to="loc_biib_AttainmentOfPerformanceCriteriain2011Member_1ADE04A9800A9514912B98B5C4EDD51D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_E91EEEBCAE1706E488B498B5C4EDF6CC" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_98E427C7314B6526C91B98B5C4EA7F3A" xlink:to="loc_us-gaap_AwardTypeAxis_E91EEEBCAE1706E488B498B5C4EDF6CC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08CA3F9AB21918A8179098B5C4EDCAC6_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_E91EEEBCAE1706E488B498B5C4EDF6CC" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08CA3F9AB21918A8179098B5C4EDCAC6_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08CA3F9AB21918A8179098B5C4EDCAC6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_E91EEEBCAE1706E488B498B5C4EDF6CC" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08CA3F9AB21918A8179098B5C4EDCAC6" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MarketStockUnitsMember" xlink:label="loc_biib_MarketStockUnitsMember_191DD044B4A36E54C5D898B5C4ED9AFC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08CA3F9AB21918A8179098B5C4EDCAC6" xlink:to="loc_biib_MarketStockUnitsMember_191DD044B4A36E54C5D898B5C4ED9AFC" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_CashSettledPerformanceSharesMember" xlink:label="loc_biib_CashSettledPerformanceSharesMember_992B527D01AE4C4F8C4E98B5C4ED6301" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08CA3F9AB21918A8179098B5C4EDCAC6" xlink:to="loc_biib_CashSettledPerformanceSharesMember_992B527D01AE4C4F8C4E98B5C4ED6301" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PerformanceSharesMember" xlink:label="loc_us-gaap_PerformanceSharesMember_D04701A8F3E67C2FC84B98B5C4EDDD87" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08CA3F9AB21918A8179098B5C4EDCAC6" xlink:to="loc_us-gaap_PerformanceSharesMember_D04701A8F3E67C2FC84B98B5C4EDDD87" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_TimeVestedRestrictedStockUnitsMember" xlink:label="loc_biib_TimeVestedRestrictedStockUnitsMember_4FD05C4E37D1A693148498B5C4EE526F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08CA3F9AB21918A8179098B5C4EDCAC6" xlink:to="loc_biib_TimeVestedRestrictedStockUnitsMember_4FD05C4E37D1A693148498B5C4EE526F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NumberOfSharesOfCommonStockForIssuance" xlink:label="loc_biib_NumberOfSharesOfCommonStockForIssuance_2B0DF2A4D268F276B56198B5C4EEC811" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_biib_NumberOfSharesOfCommonStockForIssuance_2B0DF2A4D268F276B56198B5C4EEC811" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_RatioOfNumberOfSharesReservedUnderPlan" xlink:label="loc_biib_RatioOfNumberOfSharesReservedUnderPlan_597416A45ACED01D19E098B5C4EE28E8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_biib_RatioOfNumberOfSharesReservedUnderPlan_597416A45ACED01D19E098B5C4EE28E8" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NumberOfEqualAnnualIncrements" xlink:label="loc_biib_NumberOfEqualAnnualIncrements_B9A834D8514C3AE51A7998B5C4EEB50B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_biib_NumberOfEqualAnnualIncrements_B9A834D8514C3AE51A7998B5C4EEB50B" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent" xlink:label="loc_biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent_8258BA110B8D8B04771798B5C4EE0DAE" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_biib_MinimumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent_8258BA110B8D8B04771798B5C4EE0DAE" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent" xlink:label="loc_biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent_E5BEAF446BB760BEDF6598B5C4EE3FBF" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_biib_MaximumTargetNumberOfMarketStockUnitsGrantedBasedOnStockPerformanceInPercent_E5BEAF446BB760BEDF6598B5C4EE3FBF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_3D581C856263EF9AA89598B5C4EE478F" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_3D581C856263EF9AA89598B5C4EE478F" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_EmployeeServiceSharebasedCompensationSharesUsedtoSettleAwards" xlink:label="loc_biib_EmployeeServiceSharebasedCompensationSharesUsedtoSettleAwards_83EEACD96D8765AC10D898B5C4EED193" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_biib_EmployeeServiceSharebasedCompensationSharesUsedtoSettleAwards_83EEACD96D8765AC10D898B5C4EED193" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_NumberOfDaysForCalculationOfAverageClosingStockPrice" xlink:label="loc_biib_NumberOfDaysForCalculationOfAverageClosingStockPrice_10B5CF33AA88725F456F98B5C4EED138" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_biib_NumberOfDaysForCalculationOfAverageClosingStockPrice_10B5CF33AA88725F456F98B5C4EED138" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_E94502BB5C80D2F277FE98B5C4EF806D" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_E94502BB5C80D2F277FE98B5C4EF806D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards_EDB9D6FB8745EEB108DA98B5C4EF9473" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationCashFlowEffectCashUsedToSettleAwards_EDB9D6FB8745EEB108DA98B5C4EF9473" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_D6A9F40DE69AF084A27698B5C4EF1576" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_D6A9F40DE69AF084A27698B5C4EF1576" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_7F41815A59F143F8271398B5C4EF2D52" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C24DC59B85D36B98D0D398B5C4EED878" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_7F41815A59F143F8271398B5C4EF2D52" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsShareBasedPaymentsDetails7" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C9E22B40D2ADEE8BB4BB38A35154A97C" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_932C68CAA782F08C66EE38A351549FD9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C9E22B40D2ADEE8BB4BB38A35154A97C" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_932C68CAA782F08C66EE38A351549FD9" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_D5D8B9FFCA6354DF295038A35154763A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_932C68CAA782F08C66EE38A351549FD9" xlink:to="loc_us-gaap_AwardTypeAxis_D5D8B9FFCA6354DF295038A35154763A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_228D3EE77ED713BCC6F838A35154F445_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_D5D8B9FFCA6354DF295038A35154763A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_228D3EE77ED713BCC6F838A35154F445_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_228D3EE77ED713BCC6F838A35154F445" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_D5D8B9FFCA6354DF295038A35154763A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_228D3EE77ED713BCC6F838A35154F445" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PerformanceSharesMember" xlink:label="loc_us-gaap_PerformanceSharesMember_0E29942217448DF67B7538A351548072" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_228D3EE77ED713BCC6F838A35154F445" xlink:to="loc_us-gaap_PerformanceSharesMember_0E29942217448DF67B7538A351548072" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_5F8E4546BD0D78A908B438A35154DFA9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C9E22B40D2ADEE8BB4BB38A35154A97C" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_5F8E4546BD0D78A908B438A35154DFA9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_47110D444F01E346372D38A35154C7A5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C9E22B40D2ADEE8BB4BB38A35154A97C" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_47110D444F01E346372D38A35154C7A5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_81819268573DF887430D38A351541C6B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C9E22B40D2ADEE8BB4BB38A35154A97C" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_81819268573DF887430D38A351541C6B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_8B55A0CC777ADA4C928038A351549415" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C9E22B40D2ADEE8BB4BB38A35154A97C" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_8B55A0CC777ADA4C928038A351549415" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/ShareBasedPaymentsTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPolicies" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_46A962654E2245A594B393224D476A77" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B4CCB43E4070121DA97493224D467AD5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_46A962654E2245A594B393224D476A77" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B4CCB43E4070121DA97493224D467AD5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_EF24D297BC275530EFE293224D466DA8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B4CCB43E4070121DA97493224D467AD5" xlink:to="loc_us-gaap_StatementGeographicalAxis_EF24D297BC275530EFE293224D466DA8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_B6E7172E9FB6C9D7012793224D46E44E_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_EF24D297BC275530EFE293224D466DA8" xlink:to="loc_us-gaap_SegmentGeographicalDomain_B6E7172E9FB6C9D7012793224D46E44E_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_B6E7172E9FB6C9D7012793224D46E44E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_EF24D297BC275530EFE293224D466DA8" xlink:to="loc_us-gaap_SegmentGeographicalDomain_B6E7172E9FB6C9D7012793224D46E44E" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_SpainandPortugalMember" xlink:label="loc_biib_SpainandPortugalMember_7382CD86A808443C9CD093224D468088" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_B6E7172E9FB6C9D7012793224D46E44E" xlink:to="loc_biib_SpainandPortugalMember_7382CD86A808443C9CD093224D468088" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MajorDistributorAxis" xlink:label="loc_biib_MajorDistributorAxis_CF05B3CB6FB3E1AFF4E493224D461A39" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B4CCB43E4070121DA97493224D467AD5" xlink:to="loc_biib_MajorDistributorAxis_CF05B3CB6FB3E1AFF4E493224D461A39" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MajorDistributorDomain" xlink:label="loc_biib_MajorDistributorDomain_D19608FF0747B56D040093224D469412_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_biib_MajorDistributorAxis_CF05B3CB6FB3E1AFF4E493224D461A39" xlink:to="loc_biib_MajorDistributorDomain_D19608FF0747B56D040093224D469412_default" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_MajorDistributorDomain" xlink:label="loc_biib_MajorDistributorDomain_D19608FF0747B56D040093224D469412" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_biib_MajorDistributorAxis_CF05B3CB6FB3E1AFF4E493224D461A39" xlink:to="loc_biib_MajorDistributorDomain_D19608FF0747B56D040093224D469412" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DistributorOneMember" xlink:label="loc_biib_DistributorOneMember_1B9F233B92ECCA027B4893224D46F4D4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MajorDistributorDomain_D19608FF0747B56D040093224D469412" xlink:to="loc_biib_DistributorOneMember_1B9F233B92ECCA027B4893224D46F4D4" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_DistributorTwoMember" xlink:label="loc_biib_DistributorTwoMember_BE97007B48D86CCBC8C593224D46DDB5" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_biib_MajorDistributorDomain_D19608FF0747B56D040093224D469412" xlink:to="loc_biib_DistributorTwoMember_BE97007B48D86CCBC8C593224D46DDB5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_3ED29BDE91B0021A2F6493224D472B91" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B4CCB43E4070121DA97493224D467AD5" xlink:to="loc_us-gaap_RangeAxis_3ED29BDE91B0021A2F6493224D472B91" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_1D50EB45A40490755A2393224D47B0F8_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_3ED29BDE91B0021A2F6493224D472B91" xlink:to="loc_us-gaap_RangeMember_1D50EB45A40490755A2393224D47B0F8_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_1D50EB45A40490755A2393224D47B0F8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_3ED29BDE91B0021A2F6493224D472B91" xlink:to="loc_us-gaap_RangeMember_1D50EB45A40490755A2393224D47B0F8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_DC21457430B0BED56D2593224D477B76" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_1D50EB45A40490755A2393224D47B0F8" xlink:to="loc_us-gaap_MinimumMember_DC21457430B0BED56D2593224D477B76" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_A5452D4CF061D8C7CF0893224D4715E1" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_1D50EB45A40490755A2393224D47B0F8" xlink:to="loc_us-gaap_MaximumMember_A5452D4CF061D8C7CF0893224D4715E1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_3FAF792965252BA5FD8993224D476AA5" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_B4CCB43E4070121DA97493224D467AD5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_3FAF792965252BA5FD8993224D476AA5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_D80420BD8E99199C49DD93224D47ABCA_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_3FAF792965252BA5FD8993224D476AA5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_D80420BD8E99199C49DD93224D47ABCA_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_D80420BD8E99199C49DD93224D47ABCA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_3FAF792965252BA5FD8993224D476AA5" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_D80420BD8E99199C49DD93224D47ABCA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BuildingMember" xlink:label="loc_us-gaap_BuildingMember_F6FC6A92C38C6118E51193224D47BE04" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_D80420BD8E99199C49DD93224D47ABCA" xlink:to="loc_us-gaap_BuildingMember_F6FC6A92C38C6118E51193224D47BE04" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_592C85B726B8154E5E3893224D47D094" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_D80420BD8E99199C49DD93224D47ABCA" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_592C85B726B8154E5E3893224D47D094" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MachineryAndEquipmentMember" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_74A7217FED99D99C460E93224D47991A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_D80420BD8E99199C49DD93224D47ABCA" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_74A7217FED99D99C460E93224D47991A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:label="loc_us-gaap_ComputerEquipmentMember_C77E064D4342DB0D363093224D47B055" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_D80420BD8E99199C49DD93224D47ABCA" xlink:to="loc_us-gaap_ComputerEquipmentMember_C77E064D4342DB0D363093224D47B055" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_8FDE3AC732279BA4AB8493224D47304D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_46A962654E2245A594B393224D476A77" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_8FDE3AC732279BA4AB8493224D47304D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsReceivableNetNoncurrent" xlink:label="loc_us-gaap_AccountsReceivableNetNoncurrent_D86F647C9562E54C0D4E93224D47F020" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_46A962654E2245A594B393224D476A77" xlink:to="loc_us-gaap_AccountsReceivableNetNoncurrent_D86F647C9562E54C0D4E93224D47F020" xlink:type="arc" />
    <loc xlink:href="biib-20151231.xsd#biib_PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables" xlink:label="loc_biib_PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables_F58901727424061506FF93224D47386C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_46A962654E2245A594B393224D476A77" xlink:to="loc_biib_PercentageReceivablesOfWholesaleDistributorAccountedInConsolidatedReceivables_F58901727424061506FF93224D47386C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_4F13D15A1F955C16DAD693224D474E44" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_46A962654E2245A594B393224D476A77" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_4F13D15A1F955C16DAD693224D474E44" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="extended" />
  <definitionLink xlink:role="http://www.biogenidec.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:type="extended" />
</linkbase>
