Exhibit 5.1



                                HALE AND DORR LLP
                                Counselors at Law
                                  haledorr.com
                        60 State Street, Boston, MA 02109
                         617-526-6000, Fax 617-526-5000



                                             May 13, 2003



DoubleClick Inc.
450 West 33rd Street
New York, New York, 10001



         Re:      DoubleClick Inc. 401(k) Plan
                  ----------------------------

Ladies and Gentlemen:

         We have assisted in the preparation of a Registration Statement on Form
S-8 (the "Registration Statement") to be filed with the Securities and Exchange
Commission (the "Commission") under the Securities Act of 1933, as amended (the
"Securities Act"), relating to an aggregate of 350,000 shares of common stock,
$0.001 par value per share (the "Shares"), of DoubleClick Inc., a Delaware
corporation (the "Company"), issuable under the Company's 401(k) Plan (the
"Plan").

         We have examined the Certificate of Incorporation and By-Laws of the
Company, each as amended and restated to date, and originals, or copies
certified to our satisfaction, of all pertinent records of the meetings of the
directors and stockholders of the Company, the Registration Statement and such
other documents relating to the Company as we have deemed material for the
purposes of this opinion.

         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 certified, photostatic or other copies, the authenticity of the originals
of any such documents and the legal competence of all signatories to such
documents.

         We assume that the appropriate action will be taken, prior to the offer
and sale of the Shares in accordance with the Plan, 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 General Corporation Law of the State of Delaware and
the federal laws of the United States of America.

         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.

         Based on the foregoing, we are of the opinion that the Shares have been
duly authorized for issuance and, when the Shares are issued and paid for in
accordance with the terms and conditions of the Plan, the Shares will be validly
issued, fully paid and nonassessable.

         We hereby consent to the filing of this opinion with the Commission in
connection with the Registration Statement in accordance with the requirements
of Item 601(b)(5) of Regulation S-K under the Securities Act. 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,

                                                     /s/ Hale and Dorr LLP

                                                     HALE AND DORR LLP