EXHIBIT 5.1

                                December 23, 2004

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 23, 2004 (the
"Registration Statement"), in connection with the registration under the
Securities Act of 1933, as amended, of 12,500,000 shares of the 2004 Equity
Incentive 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 2004 Equity Incentive 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 2004
Equity Incentive Plan and pursuant to the agreements that accompany the 2004
Equity Incentive 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