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                                                                     EXHIBIT 5.1


                        [Hale and Dorr LLP Letterhead]


                                                        July 16, 1999


Engage Technologies, Inc.
100 Brickstone Square
Andover, MA  01810

         Re:   Registration Statement on Form S-1
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Ladies and Gentlemen:

         This opinion is furnished to you in connection with a Registration
Statement on Form S-1 (File No. 333-78015) (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 6,900,000 shares of Common Stock, $.01 par value per share (the "Shares"), of
Engage Technologies, Inc., a Delaware corporation (the "Company"), including
900,000 Shares issuable upon exercise of an over-allotment option granted by the
Company.

         The Shares are to be sold by the Company pursuant to an underwriting
agreement (the "Underwriting Agreement") to be entered into by and among the
Company and Goldman, Sachs & Co., Hambrecht & Quist LLC and Bear, Stearns & Co.
Inc., as representatives of the several underwriters named in the Underwriting
Agreement, the form of which has been filed as Exhibit 1 to the Registration
Statement.

         We are acting as counsel for the Company in connection with the issue
and sale by the Company of the Shares. We have examined signed copies of the
Registration Statement as filed with the Commission. We have also examined and
relied upon the Underwriting Agreement, 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
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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.

         We assume that the appropriate action will be taken, prior to the offer
and sale of the Shares in accordance with the Underwriting Agreement, 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
Delaware General Corporation Law statute and the federal laws of the United
States of America. To the extent that any other laws govern the matters as to
which we are opining herein, we have assumed that such laws are identical to the
state laws of the Commonwealth of Massachusetts, and we are expressing no
opinion herein as to whether such assumption is reasonable or correct.

         Based upon and subject to 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 Underwriting
Agreement, the Shares will be validly issued, fully paid and nonassessable.

         It is understood that this opinion is to be used by you 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,


                                                      /s/ Hale and Dorr LLP

                                                      HALE AND DORR LLP

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