[HALE AND DORR LLP LETTERHEAD]
                                                                     EXHIBIT 5.1

                                            November 20, 2003

DoubleClick Inc.
450 W. 33rd Street
New York, NY 10001

     Re: DoubleClick Inc. Deferred Compensation 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 the registration of $10,000,000 of deferred
compensation obligations (the "Obligations") of DoubleClick Inc., a Delaware
corporation (the "Company"), under the DoubleClick Inc. Deferred Compensation
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
issuance of the Obligations in accordance with the Plan, to register and qualify
the Obligations for issuance 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.

         Our opinions below are qualified to the extent that they may be subject
to or affected by (i) applicable bankruptcy, insolvency, reorganization,
fraudulent conveyance or transfer, moratorium or similar laws affecting the
rights and remedies of creditors generally, (ii) statutory



DoubleClick Inc.
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November 20, 2003

or decisional law concerning recourse by creditors to security in the absence of
notice or hearing and (iii) duties and standards imposed on creditors and
parties to contracts, including, without limitation, requirements of good faith,
reasonableness and fair dealing and (iv) general equitable principles. We
express no opinion as to the availability of any equitable or specific remedy or
to the successful assertion of any equitable defenses, inasmuch as the
availability of such remedies or the success of any equitable defenses may be
subject to the discretion of a court.

         We also express no opinion herein as to any provision of any agreement
(a) which waives any right of the Company, (b) to the effect that rights and
remedies are not exclusive, that every right or remedy is cumulative and may be
exercised in addition to or with any other right or remedy and does not preclude
recourse to one or more other rights or remedies, (c) relating to the effect of
invalidity or unenforceability of any provision of the Plan on the validity or
enforceability of any other provision thereof, (d) which is in violation of
public policy, (e) relating to indemnification and contribution with respect to
securities law matters, (f) which provides that the terms of the Plan may not be
waived or modified except in writing, (g) purporting to indemnify any person
against his, her or its own negligence or misconduct, (h) requiring the payment
of penalties (including, without limitation, liquidated damages that may be
deemed or construed to constitute penalties) or consequential damages or (i)
relating to choice of law or consent to jurisdiction.

         It is understood that this opinion is to be used only in connection
with the issuance of the Obligations 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 and subject to the foregoing, we are of the opinion that the
Obligations, when issued by the Company in the manner provided in the Plan, will
be valid and binding obligations of the Company, enforceable against the Company
in accordance with their terms.

         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,

                                        HALE AND DORR LLP