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                                                                     Exhibit 5.1





                                          March 26, 2001



Concord Communications, Inc.
600 Nickerson Road
Marlboro, Massachusetts  01752

      Re:   Registration Statement on Form S-8 Relating to 2000
            Non-Executive Employee Equity Incentive Plan of Concord
            Communications, Inc.  (the "Plan")

Dear Sir or Madam:

      Reference is made to the above-captioned Registration Statement on Form
S-8 (the "Registration Statement") filed by Concord Communications, Inc. (the
"Company") on the date hereof with the Securities and Exchange Commission under
the Securities Act of 1933, as amended, relating to an aggregate of 1,500,000
shares of Common Stock, par value $.01 per share, of the Company issuable
pursuant to the Plan (the "Shares").

      We have examined, and are familiar with, and have relied as to factual
matters solely upon, copies of the Plan, the Restated Articles of Organization
and the Restated By-Laws of the Company, the minute books of the Company and
such other documents, certificates and proceedings as we have deemed necessary
for the purpose of rendering this opinion.

      Based upon and subject to the foregoing, we are of the opinion that the
Shares are duly authorized and, when issued and paid for in accordance with the
terms of the Plan and the terms of any agreement relating to any of the options
granted thereunder, will be validly issued, fully paid and nonassessable.

      We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement.

                                          Very truly yours,

                                           /s/ Testa, Hurwitz & Thibeault, LLP

                                          TESTA, HURWITZ & THIBEAULT, LLP