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  September 30, 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