EXHIBIT 5.1

                                  [Letterhead]


                                December 26, 2001




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 December 21, 2001 (the
"Registration Statement"), in connection with the registration under the
Securities Act of 1933, as amended, of an additional 5,610,752 shares of
KLA-Tencor Common Stock reserved for issuance under the Restated 1982 Stock
Option Plan, as amended (the "1982 Plan") and 2,000,000 shares of KLA-Tencor
Common Stock reserved for issuance under the Amended and Restated 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