<PAGE>
                                                                     Exhibit 5.1

                                                                      WILMERHALE


                                                             +1 617 526 6000 (t)
                                                             +1 617 526 5000 (f)
                                                                  wilmerhale.com

March 27, 2007

Akamai Technologies, Inc.
8 Cambridge Center
Cambridge, MA 02142


Re: Registration Statement on Form S-3



Ladies and Gentlemen:

This opinion is furnished to you in connection with a Registration Statement on
Form S-3 (the "Registration Statement") filed with the Securities and Exchange
Commission (the "Commission") under the Securities Act of 1933, as amended (the
"Securities Act"), for the registration of an aggregate of 2,785,034 shares of
Common Stock, $0.01 par value per share (the "Shares"), of Akamai Technologies,
Inc., a Delaware corporation (the "Company"). All of the Shares are being
registered on behalf of certain stockholders of the Company (the "Selling
Stockholders").

We are acting as counsel for the Company in connection with the registration for
resale of the Shares. We have examined signed copies of the Registration
Statement filed with the Commission. We have also examined and relied upon
minutes of meetings of the stockholders and the Board of Directors of the
Company as provided to us by the Company, stock record books of the Company as
provided to us by the Company, the Certificate of Incorporation and By-Laws of
the Company, each as restated and/or amended to date, and such other documents
as we have deemed necessary for purposes of rendering the opinions hereinafter
set forth.

In our examination of the foregoing documents, we have assumed the genuineness
of all signatures, the authenticity of all documents submitted to us as
originals, the conformity to original documents of all documents submitted to us
as copies, the authenticity of the originals of such latter documents and the
legal competence of all signatories to such documents.

Our opinion below, insofar as it relates to the Selling Stockholders' shares
being fully paid, is based solely on a certificate of the Vice President and
General Counsel of the Company confirming the Company's receipt of the
consideration called for by the applicable resolutions authorizing the issuance
of such shares.

We assume that the appropriate action will be taken, prior to the offer and sale
of the Shares, to register and qualify the Shares for sale under all applicable
state securities or "blue sky" laws.

We express no opinion herein as to the laws of any state or jurisdiction other
than the state laws of the Commonwealth of Massachusetts, the General
Corporation Law of the State of Delaware and the federal laws of the United
States of America.








                   Wilmer Cutler Pickering Hale and Dorr LLP,
                  60 State Street, Boston, Massachusetts 02109

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        Northern Virginia   Oxford   Palo Alto   Waltham   Washington

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                                                                     WILMERHALE

March 27, 2007
Page 2

Based upon and subject to the foregoing, we are of the opinion that the Shares
have been duly authorized and are validly issued, fully paid and nonassessable.

It is understood that this opinion is to be used only in connection with the
offer and sale of the Shares while the Registration Statement is in effect.

Please note that we are opining only as to the matters expressly set forth
herein, and no opinion should be inferred as to any other matters. This opinion
is based upon currently existing statutes, rules, regulations and judicial
decisions, and we disclaim any obligation to advise you of any change in any of
these sources of law or subsequent legal or factual developments which might
affect any matters or opinions set forth herein.

We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act and to the use of our
name therein and in the related prospectus under the caption "Legal Matters." In
giving such consent, we do not hereby admit that we are in the category of
persons whose consent is required under Section 7 of the Securities Act or the
rules and regulations of the Commission


Very truly yours,

WILMER CUTLER PICKERING
HALE AND DORR LLP



By: /s/ Susan W. Murley
   ---------------------------
   Susan W. Murley





