Exhibit 99.1
                    CERTIFICATION OF CHIEF EXECUTIVE OFFICER
                                   PURSUANT TO
                             18 U.S.C. SECTION 1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

I, Kenneth L. Schroeder, certify, pursuant to 18 U.S.C. Section 1350, as adopted
pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that the Quarterly
Report of KLA-Tencor Corporation on Form 10-Q for the fiscal quarter ended
December 31, 2002 fully complies with the requirements of Section 13(a) or 15(d)
of the Securities Exchange Act of 1934 and that information contained in such
Quarterly Report on Form 10-Q fairly presents in all material respects the
financial condition and results of operations of KLA-Tencor Corporation.

                                                    By: /s/ Kenneth L. Schroeder
                                                  ------------------------------
                                                      Name: Kenneth L. Schroeder
                                                  Title: Chief Executive Officer