Exhibit 5.1



                                  May 11, 1998



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


        RE:     REGISTRATION STATEMENT ON FORM S-3

Ladies and Gentlemen:

        We have examined the Registration Statement on Form S-3 to be filed by
you with the Securities and Exchange Commission on or about the date hereof (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, of up to 1,800,000 shares of your Common
Stock (the "Shares"). All of the Shares are issued and outstanding and may be
offered for sale for the benefit of the selling stockholders named in the
Registration Statement. We understand that the Shares are to be sold from time
to time on the Nasdaq National Market at prevailing prices or as otherwise
described in the Registration Statement. As your legal counsel, we have also
examined the proceedings taken by you in connection with the issuance of the
Shares. We assume that the consideration received by you in connection with each
issuance of Shares will include an amount in the form of cash, services rendered
or property that exceeds the greater of (i) the aggregate par value of such
Shares or (ii) the portion of such consideration determined by the Company's
Board of Directors to be "capital" for purposes of the Delaware General
Corporation Law.

        Subject to the foregoing, we are of the opinion that the Shares are
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, including the Prospectus constituting a part thereof,
and any amendments thereto.


                                      Very truly yours,

                                      WILSON SONSINI GOODRICH & ROSATI
                                      Professional Corporation


                                      /s/ WILSON SONSINI GOODRICH & ROSATI