EXHIBIT 5
 
                               HALE AND DORR LLP
                               Counsellors at Law

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



                                August 7, 1998



Peritus Software Services, Inc.
2 Federal Street
Billerica, Massachusetts 01821-3540


Ladies and Gentlemen:

     This opinion is furnished to you in connection with a Registration
Statement on Form S-3 (the "Registration Statement") filed with the Securities
and Exchange Commission (the "Commission") under the Securities Act of 1933, as
amended (the "Securities Act"), for the registration of an aggregate of
2,093,433 shares of Common Stock, $0.01 par value per share (the "Shares"), of
Peritus Software Services, Inc., a Massachusetts corporation (the "Company"),
all of which Shares will be sold by a certain stockholder of the Company (the
"Selling Stockholder").

     We are acting as counsel for the Company in connection with the
registration of the Shares by the Company for sale by the Selling Stockholder.
We have examined signed copies of the Registration Statement, as filed with the
Commission.  We have also examined and relied upon minutes of meetings of the
stockholders and the Board of Directors of the Company as provided to us by the
Company, stock record books of the Company as provided to us by the Company, the
Restated Articles of Organization and Restated By-Laws of the Company and such
other documents as we have deemed necessary for purposes of rendering the
opinions hereinafter set forth.

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

     Our opinion expressed below, insofar as it relates to the Shares being
fully paid, is based solely on a certificate of the Chief Financial Officer of
the Company.

     We assume that the appropriate action will be taken, prior to the offer and
sale of the Shares under the Registration Statement, 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 state laws of the Commonwealth of Massachusetts and the federal
laws of the United States of America. To the extent that any other laws govern
the matters as to which we are opining herein, we have assumed that such laws
are identical to the state laws of the Commonwealth of Massachusetts, and we are
expressing no opinion herein as to whether such assumption is reasonable or
correct.

 
Peritus Software Services, Inc.
August 7, 1998
Page 2



     Based upon and subject to the foregoing, we are of the opinion that the
Shares to be sold by the Selling Stockholder have been duly authorized and are
validly 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.  This opinion
is based upon currently existing statutes, rules, regulations and judicial
decisions, and we disclaim any obligation to advise you of any change in any of
these sources of law or subsequent legal or factual developments which might
affect any matters or opinions set forth herein.

     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 and to the use of our
name therein and in the related Prospectus under the caption "Legal Matters."
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