**Exhibit 31.1**

**CERTIFICATION OFCHIEF EXECUTIVE OFFICERPURSUANT TO RULE 13a-14(a), AS ADOPTEDPURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002**

I, S. BradfordAntle, certify that:

1.                                       Ihave reviewed this Annual Report on Form 10-K of SI International, Inc.;

2.                                       Basedon my knowledge, this report does not contain any untrue statement of amaterial fact or omit to state a material fact necessary to make the statementsmade, in light of the circumstances under which such statements were made, notmisleading with respect to the period covered by this report;

3.                                       Basedon my knowledge, the financial statements, and other financial informationincluded in this report, fairly present in all material respects the financialcondition, results of operations and cash flows of the registrant as of, andfor, the periods presented in this report;

4.                                       Theregistrant’s other certifying officer(s) and I are responsible for establishingand maintaining disclosure controls and procedures (as defined in Exchange ActRules 13a-15(e) and 15d-15(e))  and internal control overfinancial reporting (as defined in the Exchange ActRules 13a-15(f) and 15d-15(f)) for the registrant and have:

a)                                      Designedsuch disclosure controls and procedures, or caused such disclosure controls andprocedures to be designed under our supervision, to ensure that materialinformation relating to the registrant, including its consolidatedsubsidiaries, is made known to us by others within those entities, particularlyduring the period in which this report is being prepared;

b)                                     Designedsuch internal control over financial reporting, or caused such internal controlover financial reporting to be designed under our supervision, to providereasonable assurance regarding the reliability of financial reporting and thepreparation of financial statements for external purposes in accordance withgenerally accepted accounting principals;

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)                                     Disclosedin this report any change in the registrant’s internal control over financialreporting that occurred during the registrant’s fourth fiscal quarter that hasmaterially affected, or is reasonably likely to materially affect, theregistrant’s internal control over financial reporting; and

5.                                       Theregistrant’s other certifying officer(s) and I have disclosed, based on ourmost recent evaluation of internal control over financial reporting, to theregistrant’s auditors and the audit committee of the registrant’s board ofdirectors (or persons performing the equivalent functions):

a)                                      Allsignificant deficiencies and material weaknesses in the design or operation ofinternal control over financial reporting which are reasonably likely toadversely affect the registrant’s ability to record, process, summarize andreport financial information; and

b)                                     Anyfraud, whether or not material, that involves management or other employees whohave a significant role in the registrant’s internal control over financialreporting.

Date: March 13, 2007
|        | By:        | /s/ S. Bradford Antle                     |       |       |
|--------|------------|-------------------------------------------|------------|------------|
|   |       | S. Bradford Antle                         |       |       |
|   | ** ** | **President and Chief Executive Officer** | ** ** | ** ** |

 

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**CERTIFICATION OFCHIEF FINANCIAL OFFICERPURSUANT TO RULE 13a-14(a), AS ADOPTEDPURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002**

I, Thomas E. Dunn,certify that:

1.                                       Ihave reviewed this Annual Report on Form 10-K of SIInternational, Inc.;

2.                                       Basedon my knowledge, this report does not contain any untrue statement of amaterial fact or omit to state a material fact necessary to make the statementsmade, in light of the circumstances under which such statements were made, notmisleading with respect to the period covered by this report;

3.                                       Basedon my knowledge, the financial statements, and other financial informationincluded in this report, fairly present in all material respects the financialcondition, results of operations and cash flows of the registrant as of, andfor, the periods presented in this report;

4.                                       Theregistrant’s other certifying officer(s) and I are responsible for establishingand maintaining disclosure controls and procedures (as defined in Exchange ActRules 13a-15(e) and 15d-15(e))  and internal control overfinancial reporting (as defined in the Exchange ActRules 13a-15(f) and 15d-15(f)) for the registrant and have:

a)                                      Designedsuch disclosure controls and procedures, or caused such disclosure controls andprocedures to be designed under our supervision, to ensure that materialinformation relating to the registrant, including its consolidatedsubsidiaries, is made known to us by others within those entities, particularlyduring the period in which this report is being prepared;

b)                                     Designedsuch internal control over financial reporting, or caused such internal controlover financial reporting to be designed under our supervision, to providereasonable assurance regarding the reliability of financial reporting and thepreparation of financial statements for external purposes in accordance withgenerally accepted accounting principals;

c)                                      Evaluatedthe effectiveness of the registrant’s disclosure controls and procedures and presentedin this report our conclusions about the effectiveness of the disclosurecontrols and procedures, as of the end of the period covered by this reportbased on such evaluation; and

d)                                     Disclosedin this report any change in the registrant’s internal control over financialreporting that occurred during the registrant’s fourth fiscal quarter that hasmaterially affected, or is reasonably likely to materially affect, theregistrant’s internal control over financial reporting; and

5.                                       Theregistrant’s other certifying officer(s) and I have disclosed, based on ourmost recent evaluation of internal control over financial reporting, to theregistrant’s auditors and the audit committee of the registrant’s board ofdirectors (or persons performing the equivalent functions):

a)                                      Allsignificant deficiencies and material weaknesses in the design or operation ofinternal control over financial reporting which are reasonably likely toadversely affect the registrant’s ability to record, process, summarize andreport financial information; and

b)                                     Anyfraud, whether or not material, that involves management or other employees whohave a significant role in the registrant’s internal control over financialreporting.

Date: March 13, 2007
|        | By:        | /s/ Thomas E. Dunn                                                   |       |       |
|--------|------------|----------------------------------------------------------------------|------------|------------|
|   |       | Thomas E. Dunn                                                       |       |       |
|   | ** ** | **Executive Vice President, Chief Financial  Officer and Treasurer** | ** ** | ** ** |

 

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