EXHIBIT 5.1



                    [LETTERHEAD OF THELEN REID & PRIEST LLP]


                                September 7, 2001




Advanced Energy Industries, Inc.
1625 Sharp Point Drive
Fort Collins, CO  80525

     Re:  Advanced Energy Industries, Inc. - Registration Statement on Form S-8

Ladies and Gentlemen:

     We have acted as counsel for Advanced Energy Industries, Inc., a Delaware
corporation (the "Company"), in connection with the preparation of the
Registration Statement on Form S-8 relating to the issuance and sale of
2,500,000 shares of common stock, $0.001 par value, of the Company ("Common
Stock"), pursuant to the Company's 1995 Stock Option Plan (the "Plan").

     Please be advised that we are of the opinion that the Common Stock to be
offered and sold by the Company, when issued and paid for in the manner
contemplated by the Plan, will be legally issued, fully paid and non-assessable.

     We are members of the bar of the State of California and we express no
opinion as to the laws of any state or jurisdiction other than the federal laws
of the United States and the corporate laws of the State of Delaware.

     We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as an exhibit to the Registration Statement on Form S-8.

                                             Very truly yours,

                                             /s/ THELEN REID & PRIEST LLP