0001107414 2011-05-01 2012-04-30 0001107414 ssiit:S000010700Member ssiit:InstitutionalClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000015022Member ssiit:InstitutionalClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000010701Member ssiit:InstitutionalClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000019117Member ssiit:InstitutionalClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000019118Member ssiit:InstitutionalClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000010700Member ssiit:InvestmentClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000015022Member ssiit:InvestmentClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000010701Member ssiit:InvestmentClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000019117Member ssiit:InvestmentClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000019118Member ssiit:InvestmentClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000010700Member ssiit:ServiceClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000015022Member ssiit:ServiceClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000010701Member ssiit:ServiceClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000019117Member ssiit:ServiceClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000019118Member ssiit:ServiceClassMember 2011-05-01 2012-04-30 0001107414 ssiit:S000010700Member ssiit:ClassMSharesMember 2011-05-01 2012-04-30 2012-04-30 Other 2011-12-31 STATE STREET INSTITUTIONAL INVESTMENT TRUST 0001107414 2012-10-01 2012-10-01 false STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSIXX) STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (SSTXX) STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVMXX) STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRIXX) STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPIXX) INSTITUTIONAL CLASS The following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, and 14:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> The following language is hereby added to footnote 2 on the table titled, Annual Fund Operating Expenses on pages 10 and 17: The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSVXX) STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TFVXX) STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVVXX) STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRVXX) STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPVXX) INVESTMENT CLASS The following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, and 15:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> The following language is hereby added to footnote 2 on the table titled, Annual Fund Operating Expenses on pages 11 and 18: The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (LRSXX) STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TASXX) STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVSXX) STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TYSXX) STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPSXX) SERVICE CLASS The following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, 10, 13 and 16:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSLXX) CLASS M SHARES The following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on page 3:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(3) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSIXX) INSTITUTIONAL CLASS The following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, and 14:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (SSTXX)INSTITUTIONAL CLASS The following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, and 14:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVMXX)INSTITUTIONAL CLASS The following language is hereby added to footnote 2 on the table titled, Annual Fund Operating Expenses on pages 10 and 17: The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRIXX)INSTITUTIONAL CLASS The following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, and 14:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPIXX)INSTITUTIONAL CLASS The following language is hereby added to footnote 2 on the table titled, Annual Fund Operating Expenses on pages 10 and 17: The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSVXX)INVESTMENT CLASSThe following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, and 15:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TFVXX)INVESTMENT CLASSThe following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, and 15:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVVXX)INVESTMENT CLASSThe following language is hereby added to footnote 2 on the table titled, Annual Fund Operating Expenses on pages 11 and 18: The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TRVXX)INVESTMENT CLASSThe following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, and 15:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPVXX)INVESTMENT CLASSThe following language is hereby added to footnote 2 on the table titled, Annual Fund Operating Expenses on pages 11 and 18: The Adviser may also voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (LRSXX)SERVICE CLASSThe following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, 10, 13 and 16:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TAX FREE MONEY MARKET FUND (TASXX)SERVICE CLASSThe following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, 10, 13 and 16:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL U.S. GOVERNMENT MONEY MARKET FUND (GVSXX)SERVICE CLASSThe following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, 10, 13 and 16:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TREASURY MONEY MARKET FUND (TYSXX)SERVICE CLASSThe following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, 10, 13 and 16:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL TREASURY PLUS MONEY MARKET FUND (TPSXX)SERVICE CLASSThe following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on pages 3, 7, 10, 13 and 16:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(2) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table> STATE STREET INSTITUTIONAL INVESTMENT TRUST Supplement Dated October 1, 2012 to the Prospectus Dated April 30, 2012 STATE STREET INSTITUTIONAL LIQUID RESERVES FUND (SSLXX) CLASS M SHARES The following footnote is hereby inserted after Total Annual Fund Operating Expenses on the table titled, Annual Fund Operating Expenses on page 3:  <table style="BORDER-COLLAPSE:COLLAPSE" border="0" cellpadding="0" cellspacing="0" width="100%"><tr><td width="4%"> </td><td width="4%" valign="top" align="left">(3) </td><td align="left" valign="top">The Funds investment adviser, SSgA Funds Management, Inc. (the Adviser or SSgA FM), may voluntarily reduce all or a portion of its fees and/or reimburse expenses of the Fund to the extent necessary to avoid negative yield (the Voluntary Reduction) which may vary from time to time in the Advisers sole discretion. The Fund has agreed, subject to certain limitations, to reimburse the Adviser for the full dollar amount of any Voluntary Reduction incurred after October 1, 2012. The Adviser may, in its sole discretion, irrevocably waive receipt of any or all reimbursement amounts due from the Fund, without limitation. </td></tr></table>