Exhibit 31.1

CERTIFICATION OF CHIEF EXECUTIVE OFFICER  PURSUANT TO RULE 13a-14(a), AS ADOPTED  PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002

I,
Ray J. Oleson, certify that:

1.    I
have reviewed this annual report on Form 10-K of SI International, Inc.;

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

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

4.    The
registrant's other certifying officer(s) and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act
Rules 13a-15(e) and 15d-15(e)) for the registrant and have:



5.    The
registrant's other certifying officer(s) and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to the registrant's
auditors and the audit committee of the registrant's board of directors (or persons performing the equivalent functions):



Date:
March 10, 2004

| |  | By: |  | /s/ Ray J. OlesonRay J. Oleson Chairman of the Board and
Chief Executive Officer
| |  | --- |  | --------------------------------------------------------------------------------

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CERTIFICATION OF CHIEF FINANCIAL OFFICER  PURSUANT TO 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 this annual report on Form 10-K of SI International, Inc.;

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

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

4.    The
registrant's other certifying officer(s) and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Exchange Act
Rules 13a-15(e) and 15d-15(e)) for the registrant and have:



5.    The
registrant's other certifying officer(s) and I have disclosed, based on our most recent evaluation of internal control over financial reporting, to the registrant's
auditors and the audit committee of the registrant's board of directors (or persons performing the equivalent functions):



Date:
March 10, 2004

| |  | By: |  | /s/ Thomas E. DunnThomas E. Dunn Executive Vice President,
Chief Financial Officer and Treasurer
| |  | --- |  | ------------------------------------------------------------------------------------------------

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