EXHIBIT 5.1

                                  [Letterhead]

                               November 13, 2000





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

     RE:  REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

     I have examined the Registration Statement on Form S-8 to be filed with the
Securities and Exchange Commission on or about November 13, 2000 (the
"Registration Statement"), in connection with the registration under the
Securities Act of 1933, as amended, of an additional 5,616,391 shares of
KLA-Tencor Common Stock reserved for issuance under the 1982 Stock Option Plan,
as amended (the "1982 Plan") and 1,500,000 shares of KLA-Tencor Common Stock
reserved for issuance under the 1997 Employee Stock Purchase Plan (the "1997
Plan"). Such shares of Common Stock are referred to herein as the "Shares." As
legal counsel, I 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 1982 Plan and 1997 Plan.

     It is my opinion that, when issued and sold in compliance with applicable
prospectus delivery requirements and in the manner referred to in the 1982 Plan
and 1997 Plan and pursuant to the agreements which accompany the 1982 Plan and
1997 Plan, the Shares will be legally and validly issued, fully paid and
non-assessable.

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

                                       Sincerely,

                                       /s/ Stuart J. Nichols
                                       -------------------------------
                                       Stuart J. Nichols
                                       Vice President, General Counsel
                                       KLA-Tencor Corporation