**Exhibit 31.1**

**CERTIFICATION OF CHIEF EXECUTIVEOFFICER**

**PURSUANT TO RULE 13a-14(a) ASADOPTED PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002**

 

I, S. Bradford Antle, certify that:

 

1.  I have reviewed this quarterly report on Form 10-Q of SIInternational, Inc.;

 

2.  Based on my knowledge, this report does not contain anyuntrue statement of a material fact or omit to state a material fact necessaryto make the statements made, in light of the circumstances under which suchstatements were made, not misleading with respect to the period covered by thisreport;

 

3.  Based on my knowledge, the financial statements, andother financial information included in this report, fairly present in allmaterial respects the financial condition, results of operations and cash flowsof the registrant as of, and for, the periods presented in this report;

 

4.  The registrant’s other certifying officer(s) and I areresponsible for establishing and maintaining disclosure controls and procedures(as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal controlover financial reporting (as defined in the Exchange Act Rules 13a-15(f) and15d-15(f)) for the registrant and have:

 

a)  Designed such disclosure controls andprocedures, or caused such disclosure controls and procedures to be designedunder our supervision, to ensure that material information relating to theregistrant, including its consolidated subsidiaries, is made known to us byothers within those entities, particularly during the period in which thisreport is being prepared;

 

b)  Designed such internal control overfinancial reporting, or caused such internal control over financial reportingto be designed under our supervision, to provide reasonable assurance regardingthe reliability of financial reporting and the preparation of financialstatements for external purposes in accordance with generally acceptedaccounting principles;

 

c)  Evaluated the effectiveness of theregistrant’s disclosure controls and procedures and presented in this reportour conclusions about the effectiveness of the disclosure controls andprocedures, as of the end of the period covered by this report based on suchevaluation; and

 

d)  Disclosed in this report any change inthe registrant’s internal control over financial reporting that occurred duringthe registrant’s most recent fiscal quarter that has materially affected, or isreasonably likely to materially affect, the registrant’s internal control overfinancial reporting; and

 

5.  The registrant’s other certifying officer(s) and I havedisclosed, based on our most recent evaluation of internal control overfinancial reporting, to the registrant’s auditors and the audit committee ofthe registrant’s board of directors (or persons performing the equivalentfunctions):

 

a)  All significant deficiencies andmaterial weaknesses in the design or operation of internal control overfinancial reporting which are reasonably likely to adversely affect theregistrant’s ability to record, process, summarize and report financialinformation; and

 

b)  Any fraud, whether or not material, thatinvolves management or other employees who have a significant role in theregistrant’s internal control over financial reporting

 
| November 7, 2007 |   | SI INTERNATIONAL, INC.                 |
|------------------|--------|----------------------------------------|
|             |   |                                   |
|             |   |                                   |
|             |   | /s/ S. Bradford Antle                  |
|             |   | S. Bradford Antle                      |
|             |   | President and Chief  Executive Officer |

 

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**CERTIFICATIONOF CHIEF FINANCIAL OFFICER**

**PURSUANTTO RULE 13a-14(a) AS ADOPTED PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002**

 

I, Thomas E. Dunn, certify that:

 

1.  I have reviewed thisquarterly report on Form 10-Q of SI International, Inc.;

 

2.  Based on my knowledge, this report does not contain anyuntrue statement of a material fact or omit to state a material fact necessaryto make the statements made, in light of the circumstances under which suchstatements were made, not misleading with respect to the period covered by thisreport;

 

3.  Based on my knowledge, the financial statements, andother financial information included in this report, fairly present in allmaterial respects the financial condition, results of operations and cash flowsof the registrant as of, and for, the periods presented in this report;

 

4.  The registrant’s other certifying officer(s) and I areresponsible for establishing and maintaining disclosure controls and procedures(as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal controlover financial reporting (as defined in the Exchange Act Rules 13a-15(f) and15d-15(f)) for the registrant and have:

 

a)  Designed such disclosure controls andprocedures, or caused such disclosure controls and procedures to be designedunder our supervision, to ensure that material information relating to theregistrant, including its consolidated subsidiaries, is made known to us by otherswithin those entities, particularly during the period in which this report isbeing prepared;

 

b)  Designed such internal control overfinancial reporting, or caused such internal control over financial reportingto be designed under our supervision, to provide reasonable assurance regardingthe reliability of financial reporting and the preparation of financialstatements for external purposes in accordance with generally acceptedaccounting principles;

 

c)  Evaluatedthe effectiveness of the registrant’s disclosure controls and procedures andpresented in this report our conclusions about the effectiveness of thedisclosure controls and procedures, as of the end of the period covered by thisreport based on such evaluation; and

 

d)  Disclosed in this report any change inthe registrant’s internal control over financial reporting that occurred duringthe registrant’s most recent fiscal quarter that has materially affected, or isreasonably likely to materially affect, the registrant’s internal control overfinancial reporting; and

 

5.  The registrant’s other certifying officer(s) and I havedisclosed, based on our most recent evaluation of internal control overfinancial reporting, to the registrant’s auditors and the audit committee ofthe registrant’s board of directors (or persons performing the equivalentfunctions):

 

a)  All significant deficiencies andmaterial weaknesses in the design or operation of internal control overfinancial reporting which are reasonably likely to adversely affect theregistrant’s ability to record, process, summarize and report financialinformation; and

 

b)  Any fraud, whether or not material, thatinvolves management or other employees who have a significant role in theregistrant’s internal control over financial reporting.

 
| November 7, 2007 |   | SI INTERNATIONAL, INC.                 |
|------------------|--------|----------------------------------------|
|             |   |                                   |
|             |   |                                   |
|             |   | /s/ Thomas E. Dunn                     |
|             |   | Thomas E. Dunn                         |
|             |   | Executive Vice President,              |
|             |   | Chief Financial Officer  and Treasurer |

 

 

 

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