UNDERWRITING AGREEMENT

Exhibit 1.1

Biogen Idec Inc.

6.000% Senior Notes due 2013

6.875% Senior Notes due 2018

Underwriting Agreement

February 28, 2008

Goldman, Sachs & Co.
Merrill Lynch & Co.
Merrill Lynch, Pierce, Fenner & Smith Incorporated
As representatives of the several Underwriters
named in Schedule I hereto,

c/o Goldman, Sachs & Co.,
      85 Broad Street
      New York, New York 10004
-and-
      Merrill Lynch & Co.
      Merrill Lynch, Pierce, Fenner & Smith Incorporated
      4 World Financial Center
      New York, NY 10018

Ladies and Gentlemen:

Biogen Idec Inc., a Delaware corporation (the “Company”), proposes, subject to the terms and conditions stated herein, to issue and sell to Merrill Lynch & Co., Merrill Lynch, Pierce, Fenner & Smith Incorporated (“Merrill Lynch”) and Goldman, Sachs & Co. and each of the other Underwriters named in Schedule I hereto (the “Underwriters”), for whom Merrill Lynch and Goldman, Sachs & Co. are acting as representatives (in such capacity, the “Representatives”), the respective amounts set forth in Schedule I of $450,000,000 aggregate principal amount of the Company’s 6.000% Senior Notes due March 1, 2013 (the “2013 Notes”) and $550,000,000 aggregate principal amount of the Company’s 6.875% Senior Notes due March 1, 2018 (the “2018 Notes” and, together with the 2013 Notes, the “Securities”).

1. The Company represents and warrants to, and agrees with, each of the Underwriters that:

     (a) An “automatic shelf registration statement” as defined under Rule 405 under the Securities Act of 1933, as amended (the “Act”) on Form S-3 (File No. 333-149379 in respect of the Securities has been filed with the Securities and Exchange Commission (the “Commission”) not earlier than three years prior to the date hereof; such registration statement, and any post-effective amendment thereto, became effective on filing; and no stop order suspending the effectiveness of such registration statement or any part thereof has been issued and, to the Company’s knowledge, no proceeding for that purpose has been initiated or threatened by the Commission, and no notice of objection of the Commission to the use of such registration statement or any post-effective amendment thereto pursuant to Rule 401(g)(2) under the Act has been received by the Company (the base prospectus filed as part of such registration statement, in the form in which it has most recently been filed with the Commission on or prior to the date of this Agreement, is hereinafter called the “Basic Prospectus”; any preliminary prospectus (including any preliminary prospectus supplement) relating to the Securities filed with the Commission pursuant to Rule 424(b) under the Act is hereinafter called a “Preliminary Prospectus”; the various parts of such registration statement, including all exhibits thereto but excluding Form T-1 and including any prospectus supplement relating to the Securities that is filed with the Commission and deemed by virtue of Rule 430B to be part of such registration statement, each as amended at the time such part of the registration statement became effective, are hereinafter collectively called the “Registration Statement”; the Basic Prospectus, as amended and supplemented immediately prior to the Applicable Time (as defined in Section 1(c) hereof), is hereinafter called the “Pricing Prospectus”; the form of the final prospectus relating to the Securities filed with the Commission pursuant to Rule 424(b) under the Act in accordance with Section 5(a) hereof is hereinafter called the “Prospectus”; any reference herein to the Basic Prospectus, the Pricing Prospectus, any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include the documents incorporated by reference therein pursuant to Item 12 of Form S-3 under the Act, as of the date of such prospectus; any reference to any amendment or supplement to the Basic Prospectus, any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include any post-effective amendment to the Registration Statement, any prospectus supplement relating to the Securities filed with the Commission pursuant to Rule 424(b) under the Act and any documents filed under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and incorporated therein, in each case after the date of the Basic Prospectus,

such Preliminary Prospectus, or the Prospectus, as the case may be; any reference to any amendment to the Registration Statement shall be deemed to refer to and include any annual report of the Company filed pursuant to Section 13(a) or 15(d) of the Exchange Act after the effective date of the Registration Statement that is incorporated by reference in the Registration Statement; and any “issuer free writing prospectus” as defined in Rule 433 under the Act relating to the Securities is hereinafter called an “Issuer Free Writing Prospectus”);

     (b) No order preventing or suspending the use of any Preliminary Prospectus or any Issuer Free Writing Prospectus has been issued by the Commission, and each Preliminary Prospectus, at the time of filing thereof, conformed in all material respects to the requirements of the Act and the Trust Indenture Act of 1939, as amended (the “Trust Indenture Act”) and the rules and regulations of the Commission thereunder, and did not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided, however, that this representation and warranty shall not apply to any statements or omissions made in reliance upon and in conformity with information furnished in writing to the Company by an Underwriter through the Representatives expressly for use therein;

     (c) For the purposes of this Agreement, the “Applicable Time” is 12:15 p.m. (Eastern time) on the date of this Agreement; the Pricing Prospectus as supplemented by the final term sheet prepared and filed pursuant to Section 5(a) hereof, taken together (collectively, the “Pricing Disclosure Package”) as of the Applicable Time, did not include any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; and each Issuer Free Writing Prospectus listed on Schedule II(a) hereto does not conflict with the information contained in the Registration Statement, the Pricing Prospectus or the Prospectus and each such Issuer Free Writing Prospectus, as supplemented by and taken together with the Pricing Disclosure Package as of the Applicable Time, did not include any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided, however, that this representation and warranty shall not apply to statements or omissions made in an Issuer Free Writing Prospectus in reliance upon and in conformity with information furnished in writing to the Company by an Underwriter through the Representatives expressly for use therein;

     (d) The documents incorporated by reference in the Pricing Prospectus and the Prospectus, when they became effective or were filed with the Commission, as the case may be, conformed in all material respects to the requirements of the Act or the Exchange Act, as applicable, and the rules and regulations of the Commission thereunder, and none of such documents contained an untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary to make the statements therein not misleading; any further documents so filed and incorporated by reference in the Prospectus or any further amendment or supplement thereto, when such documents become effective or are filed with the Commission, as the case may be, will conform in all material respects to the requirements of the Act or the Exchange Act, as applicable, and the rules and regulations of the Commission thereunder and will not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading; provided, however, that this representation and warranty shall not apply to any statements or omissions made in reliance upon and in conformity with information furnished in writing to the Company by an Underwriter through the Representatives expressly for use therein; and no such documents were filed with the Commission since the Commission’s close of business on the business day immediately prior to the date of this Agreement and prior to the execution of this Agreement, except as set forth on Schedule II(b) hereto;

     (e) The Registration Statement conforms, and the Prospectus and any further amendments or supplements to the Registration Statement and the Prospectus will conform, in all material respects to the requirements of the Act and the Trust Indenture Act and the rules and regulations of the Commission thereunder and do not and will not, as of the applicable effective date as to each part of the Registration Statement and as of the applicable filing date as to the Prospectus and any amendment or supplement thereto, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading; provided, however, that this representation and warranty shall not apply to any statements or omissions made in reliance upon and in conformity with information furnished in writing to the Company by an Underwriter through the Representatives expressly for use therein;

     (f) Neither the Company nor any of its subsidiaries has sustained since the date of the latest audited financial statements included or incorporated by reference in the Pricing Prospectus any material loss or interference with its business from fire, explosion, flood or other calamity,

whether or not covered by insurance, or from any labor dispute or court or governmental action, order or decree, otherwise than as set forth or contemplated in the Pricing Prospectus; and, since the respective dates as of which information is given in the Registration Statement and the Pricing Prospectus, there has not been any change in the capital stock (other than changes due to grants, exercises, and terminations under Company incentive plans referred to in the Pricing Prospectus and in the documents incorporated by reference therein, repurchases of the Company’s common stock pursuant to an issuer tender offer completed on July 2, 2007 as set forth in the Pricing Prospectus and in the documents incorporated by reference therein and other publicly announced share repurchase programs) or long term debt of the Company or any of its subsidiaries or any material adverse change or any development that is reasonably likely to result in a material adverse change, in or affecting the general affairs, management, financial position, stockholders’ equity or results of operations of the Company and its subsidiaries taken as a whole (collectively, a “Material Adverse Effect”), otherwise than as set forth or contemplated in the Pricing Prospectus;

     (g) The Company and its subsidiaries have good and marketable title in fee simple to all real property and good and marketable title to all personal property owned by them, in each case free and clear of all liens, encumbrances and defects except such as are described in the Pricing Prospectus or such as would not reasonably be expected to have a Material Adverse Effect; and any real property and buildings held under lease by the Company and its subsidiaries are held by them under valid, subsisting and enforceable leases with such exceptions as would not reasonably be expected to have a Material Adverse Effect;

     (h) The Company has been duly incorporated and is validly existing as a corporation in good standing under the laws of the State of Delaware, with power and authority (corporate and other) to own its properties and conduct its business as described in the Pricing Prospectus, and has been duly qualified as a foreign corporation for the transaction of business and is in good standing under the laws of each other jurisdiction in which it owns or leases properties or conducts any business so as to require such qualification, or is subject to no material liability or disability by reason of the failure to be so qualified in any such jurisdiction except where failure would not reasonably be expected to have a Material Adverse Effect ; and each subsidiary of the Company has been duly incorporated and is validly existing as a corporation, limited liability company or other entity in good standing under the laws of its jurisdiction of organization;

     (i) The Company has an authorized capitalization as set forth in the Pricing Prospectus and all of the issued shares of capital stock of the Company have been duly and validly authorized and issued and are fully paid and non-assessable; and all of the issued shares of capital stock, membership interests or other capital securities of each subsidiary of the Company have been duly and validly authorized and issued, are fully paid and non-assessable and (except for directors’ qualifying shares) are owned directly or indirectly by the Company, free and clear of all liens, encumbrances, equities or claims;

     (j) The Securities have been duly authorized by the Company and, when issued and delivered pursuant to this Agreement, will have been duly executed, authenticated, issued and delivered and will constitute valid and legally binding obligations of the Company entitled to the benefits provided by the indenture dated as of February 26, 2008 (the “Base Indenture” and, together with the First Supplemental Indenture thereto to be dated as of a date on or prior to the Time of Delivery, the “Indenture”) between the Company and The Bank of New York Trust Company, N.A., as Trustee (the “Trustee”), under which they are to be issued, which, in the case of the Base Indenture, has been filed as an exhibit to the Registration Statement; the Indenture has been duly authorized by the Company and duly qualified under the Trust Indenture Act and, when executed and delivered by the Company and the Trustee, will constitute, at the Time of Delivery, a valid and legally binding instrument, enforceable against the Company in accordance with its terms, subject, as to enforcement, to bankruptcy, insolvency, reorganization and other laws of general applicability relating to or affecting creditors’ rights and to general equity principles; and the Securities and the Indenture will conform to the descriptions thereof in the Pricing Disclosure Package and the Prospectus;

     (k) The issue and sale of the Securities and the compliance by the Company with all of the provisions of the Securities, the Indenture and this Agreement and the consummation of the transactions herein and therein contemplated will not conflict with or result in a breach or violation of any of the terms or provisions of, or constitute a default under, any indenture, mortgage, deed of trust, loan agreement or other agreement or instrument to which the Company or any of its subsidiaries is a party or by which the Company or any of its subsidiaries is bound or to which any of the property or assets of the Company or any of its subsidiaries is subject, except as would not reasonably be expected to have a Material Adverse Effect, nor will such action result in any violation of the provisions of the Certificate of Incorporation or By-laws of the Company or any statute or any order, rule or regulation of any court or governmental agency or body

having jurisdiction over the Company or any of its subsidiaries or any of their properties; and no consent, approval, authorization, order, registration or qualification of or with any such court or governmental agency or body is required for the issue and sale of the Securities or the consummation by the Company of the transactions contemplated by this Agreement or the Indenture except such as have been, or prior to the Time of Delivery will be, obtained under the Act and the Trust Indenture Act and such consents, approvals, authorizations, registrations or qualifications as may be required under state securities or Blue Sky laws in connection with the purchase and distribution of the Securities by the Underwriters;

     (l) Neither the Company nor any of its subsidiaries is in default in the performance or observance of any material obligation, covenant or condition contained in any indenture, mortgage, deed of trust, loan agreement, lease or other agreement or instrument to which it is a party or by which it or any of its properties may be bound other than as would not reasonably be expected to have a Material Adverse Effect, or in violation of its organizational documents;

     (m) The statements set forth in the Pricing Prospectus and the Prospectus under the caption “Description of Notes”, insofar as they purport to constitute a summary of the terms of the Securities, under the caption “Material U.S. Federal Income Tax Considerations”, and under the caption “Underwriting”, insofar as they purport to describe the provisions of the laws and documents referred to therein, fairly summarize in all material respects such laws and documents;

     (n) Other than as set forth in the Pricing Prospectus, there are no legal or governmental proceedings pending to which the Company or any of its subsidiaries is a party or of which any property of the Company or any of its subsidiaries is the subject which, if determined adversely to the Company or any of its subsidiaries, would individually or in the aggregate have a material adverse effect on the financial position or results of operations of the Company and its subsidiaries taken as a whole; and, to the best of the Company’s knowledge, no such proceedings are threatened or contemplated by governmental authorities or threatened by others;

     (o) The Company is not and, after giving effect to the offering and sale of the Securities and the application of the proceeds thereof, will not be an “investment company”, as such term is defined in the Investment Company Act of 1940, as amended (the “Investment Company Act”);

     (p) (A) (i) At the time of filing the Registration Statement, (ii) at the time of the most recent amendment thereto for the purposes of complying with Section 10(a)(3) of the Act (whether such amendment was by post-effective amendment, incorporated report filed pursuant to Section 13 or 15(d) of the Exchange Act or form of prospectus), and (iii) at the time the Company or any person acting on its behalf (within the meaning, for this clause only, of Rule 163(c) under the Act) made any offer relating to the Securities in reliance on the exemption of Rule 163 under the Act, the Company was a “well-known seasoned issuer” as defined in Rule 405 under the Act; and (B) at the earliest time after the filing of the Registration Statement that the Company or another offering participant made a bona fide offer (within the meaning of Rule 164(h)(2) under the Act) of the Securities, the Company was not an “ineligible issuer” as defined in Rule 405 under the Act;

     (q) PricewaterhouseCoopers LLP , who have certified certain financial statements of the Company and its subsidiaries, and have audited the Company’s internal control over financial reporting, are independent registered public accountants as required by the Act and the rules and regulations of the Commission thereunder;

     (r) The Company maintains a system of internal control over financial reporting (as such term is defined in Rule 13a-15(f) under the Exchange Act) that complies with the requirements of the Exchange Act and has been designed by the Company’s principal executive officer and principal financial officer, or under their supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles. The Company’s internal control over financial reporting is effective and the Company is not aware of any material weaknesses in its internal control over financial reporting;

     (s) Since the date of the latest audited financial statements included or incorporated by reference in the Pricing Prospectus, there has been no change in the Company’s internal control over financial reporting that has materially affected, or is reasonably likely to materially affect, the Company’s internal control over financial reporting; and

     (t) The Company maintains disclosure controls and procedures (as such term is defined in Rule 13a-15(e) under the Exchange Act) that comply with the requirements of the Exchange Act; such disclosure controls and procedures have been designed to ensure that material information relating to the Company and its subsidiaries is made known to the Company’s principal executive officer and principal financial officer by

others within those entities; and such disclosure controls and procedures are effective.

     (u) Except as disclosed in the Pricing Prospectus, and except where such failure would not have a Material Adverse Effect, the Company and its subsidiaries own, possess, license or have the right to use the patents, patent rights, licenses, inventions, copyrights, know-how (including trade secrets and other unpatented and/or unpatentable proprietary or confidential information, systems or procedures), trademarks, service marks, trade names and other rights or interests in items of intellectual property as are necessary for the operation and conduct of the businesses now operated by them (the “patent and proprietary rights”); and except as disclosed in the Pricing Prospectus, the Company has not received notice of any infringement of or conflict with asserted rights of others with respect to any patent and proprietary rights, which infringement or conflict would reasonably be expected to result in a Material Adverse Effect.

     (v) Except as described in the Pricing Prospectus, the Company and its subsidiaries are in material compliance with all statutes, rules, regulations, ordinances, orders, decrees and guidance applicable to the ownership, testing, in humans or laboratory models, development, manufacture, formulation, packaging, processing, recordkeeping, use, distribution, marketing, labeling, promotion, sale, offer for sale, storage, import, export or disposal of any product manufactured or distributed by or for the Company or any of its subsidiaries, except where the failure to so comply would not, individually or in the aggregate, reasonably be expected to have a Material Adverse Effect.

     2. Subject to the terms and conditions herein set forth, the Company agrees to issue and sell to each of the Underwriters, and each of the Underwriters agrees, severally and not jointly, to purchase from the Company, the principal amount of Securities set forth opposite the name of such Underwriter in Schedule I hereto at a purchase price of 99.286% of the principal amount of the 2013 Notes and 98.534% of the principal amount of the 2018 Notes plus, in each case, accrued interest, if any, from March 4, 2008 to the Time of Delivery (as defined below) hereunder.

     3. Upon the authorization by you of the release of the Securities, the several Underwriters propose to offer the Securities for sale upon the terms and conditions set forth in the Prospectus.

     4. (a) The Securities to be purchased by each Underwriter hereunder will be represented by one or more definitive global Securities in book-entry form which will be deposited by or on behalf of the Company with The Depository

Trust Company (“DTC”) or its designated custodian. The Company will deliver the Securities to the Representatives, for the account of each Underwriter, against payment by or on behalf of such Underwriter of the purchase price therefor by wire transfer of Federal (same-day) funds to the account specified by the Company to the Representatives, by causing DTC to credit the Securities to the account of the Representatives at DTC. The Company will cause the certificates representing the Securities to be made available to the Representatives for checking at least twenty-four hours prior to the Time of Delivery (as defined below) at the office of DTC or its designated custodian (the “Designated Office”). The time and date of such delivery and payment shall be 9:30 a.m., New York City time, on March 4, 2008 or such other time and date as you and the Company may agree upon in writing. Such time and date are herein called the “Time of Delivery”.

     (b) The documents to be delivered at the Time of Delivery by or on behalf of the parties hereto pursuant to Section 8 hereof, including the cross-receipt for the Securities and any additional documents requested by the Underwriters pursuant to Section 8(i) hereof, will be delivered at the offices of Sidley Austin LLP 787 Seventh Avenue, New York, NY 10019 (the “Closing Location”), and the Securities will be delivered at the Designated Office, all at the Time of Delivery. A meeting will be held at the Closing Location at 12:00 p.m., New York City time, on the New York Business Day next preceding the Time of Delivery, at which meeting the final drafts of the documents to be delivered pursuant to the preceding sentence will be available for review by the parties hereto. For the purposes of this Section 4, “New York Business Day” shall mean each Monday, Tuesday, Wednesday, Thursday and Friday which is not a day on which banking institutions in New York City are generally authorized or obligated by law or executive order to close.

     5. The Company agrees with each of the Underwriters:

     (a) To prepare the Prospectus in a form that you have not reasonably disapproved and to file such Prospectus pursuant to Rule 424(b) under the Act not later than the Commission’s close of business on the second business day following the date of this Agreement; to make no further amendment or any supplement to the Registration Statement, the Basic Prospectus or the Prospectus prior to the Time of Delivery which shall be reasonably disapproved by you promptly after reasonable notice thereof; to advise you, promptly after it receives notice thereof, of the time when any amendment to the Registration Statement has been filed or becomes effective or any amendment or supplement to the Prospectus has been filed and to furnish you with copies thereof; to prepare a final term sheet, containing solely a description of the Securities, in a form approved by you and to file such term sheet pursuant to Rule 433(d) under the Act within the time required by such Rule; to file promptly all other material required to be filed by the Company with the Commission pursuant to Rule

433(d) under the Act; to file promptly all reports and any definitive proxy or information statements required to be filed by the Company with the Commission pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act subsequent to the date of the Prospectus and for so long as the delivery of a prospectus (or in lieu thereof, the notice referred to in Rule 173(a) under the Act) is required in connection with the offering or sale of the Securities; to advise you, promptly after it receives notice thereof, of the issuance by the Commission of any stop order or of any order preventing or suspending the use of any Preliminary Prospectus or other prospectus in respect of the Securities, of any notice of objection of the Commission to the use of the Registration Statement or any post-effective amendment thereto pursuant to Rule 401(g)(2) under the Act, of the suspension of the qualification of the Securities for offering or sale in any jurisdiction, of the initiation or threatening of any proceeding for any such purpose, or of any request by the Commission for the amending or supplementing of the Registration Statement or the Prospectus or for additional information; and, in the event of the issuance of any stop order or of any order preventing or suspending the use of any Preliminary Prospectus or other prospectus or suspending any such qualification, to promptly use its reasonable best efforts to obtain the withdrawal of such order; and in the event of any such issuance of a notice of objection, promptly to take such steps including, without limitation, amending the Registration Statement or filing a new registration statement, at its own expense, as may be necessary to permit offers and sales of the Securities by the Underwriters (references herein to the Registration Statement shall include any such amendment or new registration statement);

     (b) If required by Rule 430B(h) under the Act, to prepare a form of prospectus in a form approved by you and to file such form of prospectus pursuant to Rule 424(b) under the Act not later than may be required by Rule 424(b) under the Act; and to make no further amendment or supplement to such form of prospectus which shall be reasonably disapproved by you promptly after reasonable notice thereof;

     (c) Promptly from time to time to take such action as you may reasonably request to qualify the Securities for offering and sale under the securities laws of such jurisdictions as you may request and to comply with such laws so as to permit the continuance of sales and dealings therein in such jurisdictions for as long as may be necessary to complete the distribution of the Securities, provided that in connection therewith the Company shall not be required to qualify as a foreign corporation or to file a general consent to service of process in any jurisdiction;

     (d) Prior to 10:00 a.m., New York City time, on the New York Business Day next succeeding the date of this Agreement and from time to time, to furnish the Underwriters with written and electronic copies of the Prospectus in New York City in such quantities as you may reasonably request, and, if the delivery of a

prospectus (or in lieu thereof, the notice referred to in Rule 173(a) under the Act) is required at any time prior to the expiration of nine months after the time of issue of the Prospectus in connection with the offering or sale of the Securities and if at such time any event shall have occurred as a result of which the