EXHIBIT 5.1

                             CHOATE, HALL & STEWART
                A PARTNERSHIP INCLUDING PROFESSIONAL CORPORATIONS
                                 EXCHANGE PLACE
                                 53 STATE STREET
                           BOSTON, MASSACHUSETTS 02109
                            TELEPHONE (617) 248-5000
                            FACSIMILE (617) 248-4000
                                 TELEX 49615860


                                         January 16, 2001


Pegasystems Inc.
101 Main Street
Cambridge, MA  02142

Gentlemen:

         This opinion is delivered to you in connection with the registration
statement on Form S-8 (the "Registration Statement") to be filed on January 16,
2001 by Pegasystems Inc. (the "Company") under the Securities Act of 1933, as
amended, for registration under said Act of 2,000,000 shares (the "Additional
Shares") of common stock, $.01 par value (the "Common Stock"), of the Company.

         We are familiar with the Company's Restated Articles of Organization,
its Restated By-Laws, and its corporate minute book, as well as the Registration
Statement. We have also examined such other documents, records and certificates
and made such further investigation as we have deemed necessary for the purposes
of this opinion.

         Based upon and subject to the foregoing, we are of the opinion that the
Additional Shares to be sold by the Company under its Amended and Restated 1994
Long-Term Incentive Plan, as in effect on the date hereof, when issued against
receipt of the agreed purchase price therefor, will be legally issued, fully
paid and nonassessable.

         We understand that this opinion is to be used in connection with the
Registration Statement and consent to the filing of this opinion as an exhibit
to the Registration Statement and to all references to this Firm included
therein.

                                                     Very truly yours,

                                                     /s/ CHOATE, HALL & STEWART

                                                     CHOATE, HALL & STEWART