EXHIBIT 5.1



                               November 3, 1999

Plug Power Inc.
968 Albany-Shaker Road
Latham, New York 12110

Ladies and Gentlemen:

  Re:    Registration Statement on Form S-8
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  This opinion is delivered in our capacity as counsel to Plug Power Inc. (the
"Company") in connection with the preparation and filing with the Securities and
Exchange Commission under the Securities Act of 1933, as amended (the "Act"), of
a Registration Statement on Form S-8 (the "Registration Statement") relating to
1,000,000 shares of Common Stock, par value $.01 per share (the "Registered
Shares"), which the Company may issue pursuant to its Employee Stock Purchase
Plan (the "Plan").

  As counsel for the Company, we have examined a copy of the Plan and the
Company's Amended and Restated Certificate of Incorporation and the Company's
Amended and Restated By-laws, each as presently in effect, and such records,
certificates and other documents of the Company as we have deemed necessary or
appropriate for the purposes of this opinion.

  We are attorneys admitted to practice in The Commonwealth of Massachusetts.
We express no opinion concerning the laws of any jurisdiction other than the
laws of the United States of America, The Commonwealth of Massachusetts and the
State of Delaware.

  Based on the foregoing, we are of the opinion that upon the issuance and
delivery of the Registered Shares against payment therefor in accordance with
the terms of the Plan and any agreement thereunder, the Registered Shares will
be legally issued, fully paid and non-assessable shares of the Company's Common
Stock.

  The foregoing assumes all requisite steps will be taken to comply with the
requirements of the Act, applicable requirements of state laws regulating the
offer and sale of securities and applicable requirements of The Nasdaq Stock
Market, Inc.

  We hereby consent to being named as counsel to the Company in the Registration
Statement and to the inclusion of this opinion as an exhibit to the Registration
Statement.

                                         Very truly yours,

                                         /s/ Goodwin, Procter & Hoar LLP

                                         GOODWIN, PROCTER & HOAR LLP