EXHIBIT 5

                         Testa, Hurwitz & Thibeault, LLP
                                 125 High Street
                                Boston, MA 02110

                                                              November 15, 1999

NetScout Systems, Inc.
4 Technology Park Drive
Westford, MA  01886

Re:      REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

         We are acting as counsel for NetScout Systems, Inc., a Delaware
corporation (the "COMPANY"), in connection with the registration on a
Registration Statement on Form S-8 (the "REGISTRATION STATEMENT") under the
Securities Act of 1933, as amended, of the offer and sale of up to 4,500,000
shares (the "SHARES") of Common Stock, par value $.001 per share, of the Company
under the 1999 Stock Option and Incentive Plan (the "1999 PLAN").

         In rendering our opinion, we have examined, and are familiar with, and
have relied as to factual matters solely upon, originals or copies certified, or
otherwise identified to our satisfaction, of such documents, corporate records
or other instruments as we have deemed necessary or appropriate for the purposes
of the opinion set forth herein, including, without limitation, (a) the 1999
Plan, (b) the Company's Third Amended and Restated Certificate of Incorporation,
(c) the Company's Amended and Restated By-laws, (d) a specimen of the form of
Certificate evidencing the Shares and (e) the minute books of the Company.

         Based upon and subject to the foregoing, we are of the opinion that the
Shares are duly authorized and, when issued and delivered pursuant to the terms
of the 1999 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 to the
Registration Statement.

                                           Very truly yours,

                                           /s/ Testa, Hurwitz & Thibeault, LLP

                                           TESTA, HURWITZ & THIBEAULT, LLP