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                                                   [MAYER BROWN ROWE & MAW LOGO]

September 15, 2006                                  Mayer, Brown, Rowe & Maw LLP
                                                           71 South Wacker Drive
                                                   Chicago, Illinois  60606-4637

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CommVault Systems, Inc.
2 Crescent Place
Oceanport, NJ 07757

Re:   Registration Statement on Form S-1
      File No. 333-132550

Ladies and Gentlemen:

      We have acted as counsel to CommVault Systems, Inc., a Delaware
corporation (the "Company"), in connection with the corporate proceedings taken
and to be taken relating to the public offering by the Company and by certain
stockholders of the Company of up to 12,777,778 shares of the Company's common
stock, $0.01 par value per share (the "Common Stock"). We have also participated
in the preparation of the Company's Registration Statement on Form S-1 (File No.
333-132550) (the "Registration Statement") filed with the Securities and
Exchange Commission (the "Commission") under the Securities Act of 1933, as
amended, relating to such Shares. The Company will offer and may sell up to
6,148,148 of such shares of Common Stock pursuant to the Registration Statement
(the "Primary Shares") and up to 6,629,630 of such shares of Common Stock will
be offered and may be sold by certain selling stockholders pursuant to the
Registration Statement (the "Secondary Shares"). Certain of the Secondary Shares
will be issued to the selling stockholders upon conversion of the outstanding
shares of the Company's preferred stock (the "Conversion"). In rendering the
opinion set forth below, we have examined such corporate and other records,
instruments, certificates and documents as we considered necessary to enable us
to express this opinion.

      Based upon the foregoing, we are of the opinion that:

      1. Upon the approval of the terms of the transaction and the underwriting
arrangements by the Pricing Committee of the Board of Directors of the Company
and the Filing of the Amended and Restated Certificate of Incorporation of the
Company with the Secretary of State of the State of Delaware in the manner
contemplated in the Registration Statement, the Primary Shares will have been
duly authorized and, when the Primary Shares are delivered in accordance with
the Underwriting Agreement in substantially the form filed as Exhibit 1.1 to the
Registration Statement, will be validly issued, fully paid and non-assessable.

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CommVault Systems, Inc.
September 15, 2006
Page 2

      2. The Secondary Shares which are currently outstanding were validly
issued, fully paid and non-assessable.

      3. Upon the filing of the Amended and Restated Certificate of
Incorporation of the Company with the Secretary of State of the State of
Delaware as contemplated in the Registration Statement, the Secondary Shares
which will be issued in the Conversion will be validly issued, fully paid and
non-assessable.

      We hereby consent to the filing of this opinion as Exhibit 5 to the
Registration Statement and to the reference to this firm under the caption
"Legal Matters" in the Prospectus constituting a part of the Registration
Statement.

                                      Very truly yours,


                                      /s/ Mayer, Brown, Rowe & Maw LLP

                                      Mayer, Brown, Rowe & Maw LLP
