EXHIBIT 5.1


(Testa, Hurwitz & Thibeault, LLP Letterhead)
                                                                   June 9, 1998


Teradyne, Inc.
321 Harrison Avenue
Boston, Massachusetts 02118

      Re:     Amendment to Registration Statement on Form S-8 Relating to the
              Teradyne, Inc. 1996 Employee Stock Purchase Plan (the "1996 Plan")
              and the 1997 Employee Stock Option Plan (the "1997 Plan")
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Ladies and Gentlemen:

      Reference  is  made  to  the  above-captioned  Amendment  to  Registration
Statement on Form S-8 (the "Amendment)  filed by Teradyne,  Inc. (the "Company")
with the Securities and Exchange Commission under the Securities Act of 1933, as
amended,  relating to 8,000,000 shares of Common Stock,  $.125 par value, of the
Company (the "Shares").

      We are counsel to the Company and are familiar with the proceedings of its
stockholders  and Board of  Directors.  We have  examined  original or certified
copies of the Company's certificate of incorporation,  as amended, the Company's
by-laws,  as amended,  the corporate  records of the Company to the date hereof,
and such other certificates,  documents, records and materials as we have deemed
necessary in connection with this opinion letter.

      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, and the General Corporation Law of the State of Delaware.

      Based upon and subject to the  foregoing,  we are of the opinion  that the
Shares issued or proposed to be issued by the Company  pursuant to the 1996 Plan
and the 1997 Plan  (collectively,  the  "Plans")  will be,  upon  receipt of the
consideration  provided for in each of the Plans, validly issued, fully paid and
nonassessable after issuance of such Shares in accordance with the terms of each
of the Plans.

      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
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                                            TESTA, HURWITZ & THIBEAULT, LLP