[LETTERHEAD OF THELEN REID & PRIEST LLP]

                               October 5, 1998


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


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 common
stock,  $0.001 par value,  of the  Company  ("Common  Stock"),  pursuant  to its
assumption  of the RF Power  Products,  Inc.  1992 Stock Option  Plan,  RF Power
Products,  Inc.  1993  Non-Employee  Director  Stock  Option  Plan  and RF Power
Products,  Inc.  1995 Stock Option Plan  (collectively,  the  "Plans"),  and the
options granted thereunder.

     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   Plans,   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 federal laws of
the United States, the laws of the State of California and the corporate laws of
the State of Delaware.

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

                                                       Very truly yours,

                                                     /s/THELEN REID & PRIEST LLP
                                                    THELEN REID & PRIEST LLP