The Department of Homeland Security (DHS) estimated that as of fiscal year 2009 the total alien-non-U.S.-citizen-population was about 25.3 million, including about 10.8 million aliens without lawful immigration status. Some aliens have been convicted and incarcerated (criminal aliens). The federal government bears these incarceration costs for federal prisons and reimburses states and localities for portions of their costs through the Department of Justice's (DOJ) State Criminal Alien Assistance Program (SCAAP). GAO was asked to update its April and May 2005 reports that contained information on criminal aliens. This report addresses (1) the number and nationalities of incarcerated criminal aliens; (2) the types of offenses for which criminal aliens were arrested and convicted; and (3) the costs associated with incarcerating criminal aliens and the extent to which DOJ's methodology for reimbursing states and localities for incarcerating criminal aliens is current and relevant. GAO analyzed federal and SCAAP incarceration and cost data of criminal aliens from fiscal years 2003 through 2010, and conviction and cost data from five states that account for about 70 percent of the SCAAP criminal alien population in 2008. GAO analyzed a random sample of 1,000 criminal aliens to estimate arrest information due to the large volume of arrests...