EXHIBIT 5
                                                                       ---------

                               HALE AND DORR LLP
                               Counsellors at Law

                  60 State Street, Boston, Massachusetts 02109
                       617-526-6000  *  FAX 617-526-5000



                               September 9, 1997



Peritus Software Services, Inc.
304 Concord Road
Billerica, Massachusetts 01821-3485


        Re:     1997 Employee Stock Purchase 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 200,000 shares of common stock, $.01
par value per share (the "Shares"), of Peritus Software Services, Inc., a
Massachusetts corporation (the "Company"), issuable under the Company's 1997
Employee Stock Purchase Plan (the "Plan").

        We have examined the Restated Articles of Organization of the Company
and the Amended and Restated By-Laws of the Company, 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.

 
Peritus Software Services, Inc.
September 9, 1997
Page 2


        We express no opinion herein as to the laws of any state or jurisdiction
other than the state laws of the Commonwealth of Massachusetts 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 mattes 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
(the "Securities Act").  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 the
Securities Act or the rules and regulations of the Commission.

                                             Very truly yours,



                                             /s/ Hale and Dorr LLP

                                             HALE AND DORR LLP