EXHIBIT 5.1

                                  [LETTERHEAD]

                                  May 7, 1997



KLA-Tencor Corporation
160 Rio Robles
San Jose, CA 95134

    RE:  REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

    We have examined the Registration Statement on Form S-8 to be filed by you
with the Securities and Exchange Commission on May 7, 1997 (the "Registration
Statement") in connection with the registration under the Securities Act of
1933, as amended, of an aggregate of 3,133,335 shares of your Common Stock, par
value $0.001 per share (the "Shares") to be issued pursuant to the Tencor
Instruments Second Amended and Restated 1984 Stock Option Plan, Tencor
Instruments Amended and Restated 1993 Equity Incentive Plan, Tencor Instruments
1993 Nonemployee Directors Stock Option Plan, Tencor Instruments 1993 Employee
Stock Purchase Plan, Tencor Instruments 1993 Foreign Subsidiary Employee Stock
Purchase Plan, 1983 Employee Incentive Stock Option Plan of Prometrix
Corporation and 1993 Employee Incentive Stock Option Plan of Prometrix
Corporation (collectively, the "Plans").  As your counsel in connection with
this transaction, we have examined the proceedings taken and are familiar with
the proceedings proposed to be taken by you in connection with the issuance and
sale of the Shares pursuant to the Plans.

    It is our opinion that, when issued and sold in the manner described in the
Plans and pursuant to the agreements which accompany each grant under the
Plans, the Shares will be legally and validly issued, fully-paid and
non-assessable.

    We consent to the use of this opinion as an exhibit to the Registration
Statement, and further consent to the use of our name wherever appearing in the
Registration Statement and any amendments thereto.

                            Very truly yours,

                            WILSON SONSINI GOODRICH & ROSATI


                            /s/ WILSON SONSINI GOODRICH & ROSATI