Exhibit 32.1

CERTIFICATIONS PURSUANT TO

18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

Each of the undersigned hereby certifies, in his/her capacity as an officer of CEB Inc. (“Company”), for purposes of 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that to the best of his knowledge:

• The Quarterly Report on Form 10-Q of the Company for the quarterly period ended September 30, 2015 fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

• The information contained in such Quarterly Report on Form 10-Q fairly presents, in all material respects, the financial condition and results of operations of the Company.

/s/ Thomas L. Monahan III

Thomas L. Monahan III

Chairman of the Board of Directors and

Chief Executive Officer

November 9, 2015

/s/ Richard S. Lindahl

Richard S. Lindahl

Chief Financial Officer

November 9, 2015