1


                                                                     Exhibit 5.1



                                                  May 28, 1998

Atlantic Data Services, Inc.
One Batterymarch Park
Quincy, Massachusetts  02169

         Re:      Atlantic Data Services, Inc.
                  Registration Statement on Form S-8

Ladies and Gentlemen:

      We are acting as counsel for Atlantic Data Services, Inc., a Massachusetts
corporation (the "Company"), in connection with the registration on a
Registration Statement on Form S-8 (the "Registration Statement") under the
Securities Act of 1933, as amended, of the offer and sale of up to 2,263,010
shares of Common Stock, par value $.01 per share, of the Company (the "Shares")
to be issued upon the exercise of options under the Amended and Restated 1992
Incentive Stock Option Plan (the "1992 Plan") and the Amended and Restated 1997
Stock Plan (the "1997 Plan") and the purchase of Shares under the 1998 Employee
Stock Purchase Plan (the "Purchase Plan" and, collectively, with the 1992 Plan
and the 1997 Plan, the "Plans") (as described in the Registration Statement)
(collectively the "Options").

      In rendering our opinion, we have examined, and are familiar with, and
have relied as to factual matters solely upon, originals or copies certified, or
otherwise identified to our satisfaction, of such documents, corporate records
or other instruments as we have deemed necessary or appropriate for the purposes
of the opinion set forth herein, including, without limitation, (a) the Plans,
(b) the Company's Amended and Restated Articles of Organization, (c) the
Company's Second Amended and Restated Articles of Organization, (d) the
Company's Amended and Restated By-laws, (e) the Company's Second Amended and
Restated By-laws, (f) a specimen of the form of Certificate evidencing the
Shares, (g) the minute books and stock records of the Company, and (h) a
Certificate of the Clerk of the Company dated May 27, 1998 (without making any
independent investigation or inquiry with respect to the completeness or
accuracy of such statements contained in such Certificate).

      We are members only of the bar of the Commonwealth of Massachusetts and
are not experts in, and express no opinion regarding, the laws of any
jurisdiction other than the Commonwealth of Massachusetts and the United States
of America.

      Based upon and subject to the foregoing, we are of the opinion that the
Shares are duly authorized and, when issued and delivered pursuant to the terms
of the Options against receipt by the Company of the consideration therefor as
provided therein, will be validly issued, fully paid and nonassessable.

      We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement.

                                             Very truly yours,

                                             /s/ Testa, Hurwitz & Thibeault, LLP

                                             TESTA, HURWITZ & THIBEAULT, LLP