Exhibit 5

                            [Ropes & Gray Letterhead]


                                                   September 18, 2002


Forrester Research, Inc.
400 Technology Square
Cambridge, Massachusetts 02139

     Re:  Forrester Research, Inc.
          ------------------------

Ladies and Gentlemen:

         This opinion is furnished to you in connection with a registration
statement on Form S-8, and all exhibits thereto (the "Registration Statement"),
filed with the Securities and Exchange Commission under the Securities Act of
1933, as amended, for the registration of 300,000 shares of common stock, $.01
par value (the "Shares"), of Forrester Research, Inc., a Delaware corporation
(the "Company"). The Shares are to be issued pursuant to the exercise of options
granted under the Company's Amended and Restated 1996 Stock Option Plan for
Non-Employee Directors (the "Plan").

         We have acted as counsel for the Company and are familiar with the
actions taken by the Company in connection with the Plan. For purposes of this
opinion we have examined the Registration Statement, the Plan and such other
documents, records, certificates and other instruments as we have deemed
necessary.

         We express no opinion as to the applicability of compliance with or
effect of Federal law or the law of any jurisdiction other than The Commonwealth
of Massachusetts and the corporate laws of the State of Delaware.

         Based on the foregoing, we are of the opinion that, when the Shares
have been issued and sold in accordance with the terms of the Plan, the Shares
will be validly issued, fully paid and non-assessable.

         We hereby consent to your filing this opinion as an exhibit to the
Registration Statement.

         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.


                                                  Very truly yours,

                                                  /s/ Ropes & Gray

                                                  Ropes & Gray


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