EXHIBIT 5.1

                                November 3, 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 November 3, 2004 (the
"Registration Statement"), in connection with the registration under the
Securities Act of 1933, as amended, of 23,339 shares of the Blue29 Corporation
2003 Stock 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 Blue29 Corporation 2003
Stock 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 Blue29
Corporation 2003 Stock Incentive Plan and pursuant to the agreements that
accompany the Blue29 Corporation 2003 Stock 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