EXHIBIT 5.1




                                  [Letterhead]


                                 August 10, 1999





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 or about August 13, 1999 (the
"Registration Statement"), in connection with the registration under the
Securities Act of 1933, as amended, of an additional 2,660,462 shares of your
Common Stock (the "Shares") reserved for issuance under the Restated 1982 Stock
Option Plan, as amended (the "Stock Plan"). As your legal counsel, we have
examined the proceedings taken and proposed to be taken in connection with the
issuance, sale and payment of consideration for the Shares to be issued under
the Plan.

         It is our opinion that, when issued and sold in compliance with
applicable prospectus delivery requirements and in the manner referred to in the
Plan and pursuant to the agreements which accompany the Plan, 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.



                                        Sincerely,

                                        WILSON SONSINI GOODRICH & ROSATI
                                        Professional Corporation

                                        /s/ Wilson Sonsini Goodrich & Rosati