U.S. compacts with the freely associated states (FAS)--the Federated States of Micronesia (FSM), the Marshall Islands, and Palau--permit FAS citizens to migrate to the United States and its territories (U.S. areas) without regard to visa and labor certification requirements. Thousands of FAS citizens have migrated to U.S. areas (compact migrants)--particularly to the Commonwealth of the Northern Mariana Islands (CNMI), Guam, and Hawaii, which are defined as affected jurisdictions. In fiscal year 2004, Congress appropriated $30 million annually for 20 years to help defray affected jurisdictions' costs for migrant services (compact impact). Though not required, affected jurisdictions can report these costs to the Department of the Interior (Interior), which allocates the $30 million as impact grants in proportion to compact migrant enumerations required every 5 years. This report: (1) describes compact migration; (2) reviews enumeration approaches; (3) evaluates impact reporting; and (4) reviews Interior grants related to compact impact. GAO reviewed U.S. agency data, recent enumerations, impact reports, and grants and it also interviewed officials, employers, and migrants in the affected jurisdictions. GAO recommends that Interior assess the 2013 enumeration approach, disseminate adequate guidance on estimating compact impact, and encourage uses of grants that better address compact migrants' impact and needs. Interior generally agreed with the report but did not support the recommendation on grant uses.