1


                                HALE AND DORR LLP
                               Counsellors at Law
                  60 State Street, Boston, Massachusetts 02109
                         617-526-6000 * FAX 617-526-5000



                                  July 28, 2000



Aspen Technology, Inc.
Ten Canal Park
Cambridge, Massachusetts 02141

     Re:  Aspen Technology, Inc. 1995 Stock Option Plan
          ---------------------------------------------

Ladies and Gentlemen:

     We have assisted in the preparation of a Registration Statement on Form S-8
(the "Registration Statement") to be filed with the Securities and Exchange
Commission (the "Commission") relating to 2,690,109 shares of common stock, $.10
par value per share (the "Shares"), of Aspen Technology, Inc., a Delaware
corporation (the "Company"), issuable under the Company's Aspen Technology, Inc.
1995 Stock Option Plan (the "Plan").

     We have examined the Certificate of Incorporation of the Company and the
By-laws of the Company, each as amended, and originals, or copies certified to
our satisfaction, of all pertinent records of the meetings of the directors and
stockholders of the Company, the Registration Statement and such other documents
relating to the Company as we have deemed material for the purposes of this
opinion.

     In examination of the foregoing documents, we have assumed the genuineness
of all signatures and the authenticity of all documents submitted to us as
originals, the conformity to original documents of all documents submitted to us
as certified, photostatic or facsimile copies, the authenticity of the originals
of such latter documents and the legal competence of all signatories to such
documents.

     We assume that the appropriate action will be taken, prior to the offer and
sale of the Shares in accordance with the Plan, to register and qualify the
Shares for sale under all applicable state securities or "blue sky" laws.

     We express no opinion herein as to the laws of any state or jurisdiction
other than the Delaware General Corporation Law and the federal laws of the
United States of America.

     Based upon and subject to the foregoing, we are of the opinion that the
Company has duly authorized for issuance the Shares covered by the Registration
Statement to be issued under the Plan, as described in the Registration
Statement, and such Shares, when issued in accordance with the terms of the
Plan, will be legally issued, fully paid and nonassessable.

     It is understood that this opinion is to be used only in connection with
the offer and sale of the Shares while the Registration Statement is in effect.

     Please note that we are opining only as to the matters expressly set forth
herein, and no opinion should be inferred as to any other matters.

     We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended.
In giving such consent, we do not hereby admit that we are in the category of
persons whose consent is required under Section 7 of such Act or the rules and
regulations of the Commission.

                                       Very truly yours,


                                       HALE AND DORR LLP


                                      -9-