Congress has a long-standing interest in the criminal alien population and has supported efforts since the late 1980s to identify, detain, and remove these individuals. The Congressional Research Service (CRS) estimates that about 184,000 non-citizens were incarcerated in 2011 ( Table 2 ) and that the total non-citizen correctional population—which represent a subset of all aliens ever convicted of a crime—may be about 572,000 (see " Other Estimates of the Criminal Alien Population " below). More generally, all unauthorized immigrants within the United States are potentially subject to removal, and alien removals originating within the United States (i.e., "interior enforcement") are a basic element of immigration control. The Department of Homeland Security's (DHS) Immigration and Customs Enforcement (ICE) agency operates four key programs to identify and remove criminal aliens and other removable aliens. The Criminal Alien Program (CAP) is a screening program that identifies, detains, and initiates removal proceedings against criminal aliens, including within federal, state, and local prisons and jails. Secure Communities is an information sharing program between the Departments of Justice and Homeland Security that screens for removable aliens as people are being booked into jails. Agreements entered into pursuant to Immigration and Nationality Act (INA) §287(g) ("§287(g) agreements") allow DHS to delegate certain immigration enforcement functions to specially trained state and local law enforcement officers, under federal supervision. The National Fugitive Operations Program (NFOP) pursues known at-large criminal aliens and fugitive aliens. Funding for interior enforcement and programs targeting criminal aliens has expanded considerably since FY2005 as has the number of aliens arrested through them. Congress appropriated a total of about $567 million to ICE for these four programs in FY2013, up from $23 million in FY2004 (see " Appropriations " below). At the same time, the number of aliens arrested through ICE's major interior enforcement programs increased during these years from about 11,000 to about 270,000 ( Table 6 ). While ICE's interior enforcement programs are designed to focus in particular on criminal aliens, the majority of unauthorized aliens in the United States have not been convicted of a crime ( Figure 1 ). Some have criticized these programs because most of the aliens they identify have no criminal record or have committed only nonviolent crimes. Others note, however, that all unauthorized aliens have violated U.S. immigration law and may be subject to removal. Partly for these reasons, interior enforcement and programs targeting criminal aliens raise two key policy issues for Congress. First, when, if ever, should DHS exercise its "prosecutorial discretion" by not asserting its full enforcement authority? ICE's interior enforcement programs have greatly increased the agency's ability to identify potentially removable aliens, but the Obama Administration has argued that it only has resources to deport about 400,000 aliens per year (see " DHS Enforcement Priorities and Discretion "). The Administration has published formal guidelines to prioritize certain types of cases for enforcement and to exercise discretion in other cases. Some Members of Congress have objected to these policies, and some support legislation to limit the exercise of discretion (see " DHS Enforcement Priorities and Discretion "). A second set of questions concerns the role of state and local law enforcement agencies in immigration enforcement. While some Members of Congress and some local governments and law enforcement agencies favor a larger role for states and localities in immigration enforcement, others are concerned that such enforcement practices may undermine civil rights and may have a chilling effect on community-police relations. Certain jurisdictions have sought to limit their participation in such programs. But after initially treating Secure Communities as a voluntary federal-local partnership, the Obama Administration no longer permits local jurisdictions to "opt out" of it (see " The State and Local Role in ICE's Interior Enforcement Programs "). This report begins by defining and quantifying the criminal alien population, to the extent possible. The following sections describe current and historical programs designed in whole or in part to target this population, including CAP, Secure Communities, the §287(g) program, and NFOP. After describing how these programs function and key differences among them, the report reviews their recent appropriations history and enforcement statistics. The final sections of the report describe the controversies surrounding certain programs targeting criminal aliens—particularly the Secure Communities program and the §287(g) program—and legislative issues that may arise as a result. For over a century, U.S. immigration law has identified certain crimes that make an alien ineligible for admission to the United States and/or subject to deportation. Such crimes include crimes of "moral turpitude," crimes involving controlled substances, certain firearm offenses, and crimes related to espionage, sabotage, and related offenses. Yet the term "criminal alien" is not specifically defined in immigration law or regulation, and people use it to refer to several different types of noncitizen offenders. At the broadest level, a "criminal alien" is any noncitizen who has ever been convicted of a crime in the United States . This report adopts this broad definition unless otherwise noted. See Appendix A for a glossary of additional terms related to the criminal alien population. Not all criminal aliens are unauthorized or removable, and most removable aliens are not criminals (see Figure 1 ). Three groups of criminal aliens can be distinguished. First, the set of all criminal aliens includes both unauthorized aliens, all of whom are potentially removable, and legal aliens. Second, some criminal aliens who have been convicted of removable criminal offenses are subject to removal under the INA even if they are otherwise legally present. For example, a legal permanent resident (LPR) convicted of cocaine possession is subject to removal, but an LPR convicted of public intoxication is not. Third, a subset of these removable criminal aliens are aggravated felons , and therefore are ineligible for most forms of relief from removal and ineligible to be readmitted to the United States. As Figure 1 illustrates, all unauthorized aliens are potentially removable, indicated by cross-hatches in the figure, but the majority of unauthorized aliens have not been convicted of a crime and are not classified as criminal aliens. (Unlawful presence in the United States absent additional factors is a civil violation, not a criminal offense. ) The smaller circles in Figure 1 illustrate that some criminal aliens are removable on the basis of the specific crimes committed, and that some are unauthorized. This section presents publicly available arrest and incarceration data for criminal aliens at the federal, state, and local levels from 2001 through 2011. Following CRS's quantification of the criminal alien population, the section reviews other studies that produced comparable estimates. Appendix B describes related data issues in greater detail. Table 1 presents data from the Department of Justice (DOJ), Bureau of Justice Statistics (BJS) on the number and percentage of persons arrested for federal offenses, by citizenship status, for 2001, 2005, and 2010. Of the 179,489 persons arrested for federal offenses in 2010, 46% were not U.S. citizens and 16% had unknown citizenship status. The data in Table 1 also indicate that the proportion of noncitizens arrested for federal offenses increased across the period analyzed. Most federal arrests of noncitizen between 2001 and 2010 were for illegal entry. As Figure 2 illustrates, while federal arrests with known citizenship information increased 42%, from 109,699 to 151,456 during this period, noncitizen arrests for illegal entry—which increased noticeably after 2003—accounted for virtually all of this increase. Noncitizen arrests for all other federal offenses accounted for a declining share of all federal arrests, from 19% in 2001 to 5% in 2010. The growth of illegal entry cases caused arrests of U.S. citizens to account for a declining share of all federal arrests (from 62% in 2001 to 45% in 2010). These trends may reflect changes in enforcement and prosecution policies rather than increased noncitizen criminality. The number of Border Patrol apprehensions has always far exceeded the number of arrests for illegal entry, and DHS and DOJ have adopted policies to seek criminal charges against a higher proportion of such aliens, particularly since 2005 through Operation Streamline and related programs. Incarceration represents a second measure of the criminal alien population. Table 2 presents CRS tabulations of the total citizen and noncitizen prison and jail populations publicly reported by the Bureau of Justice Statistics for 2001 through 2011 (the most recent year for which complete data are available). Estimates of state and, especially, local incarcerated populations should be interpreted with caution for the reasons noted in Appendix B . As of June 30, 2011, a total of 2,334,381 prisoners (U.S. citizens and noncitizens) were incarcerated: 216,362 in federal prisons, 1,382,418 in state prisons, and 735,601 in local jails ( Table 2 ). According to BJS data presented in Table 2 , noncitizens comprised 7.9% of the combined federal, state, and local prisoner population in 2011, including 26.9% of the federal prison population, 5.2% of the state prison population, and 7.3% of the local jail population. As a basis for comparison, noncitizens comprised 7.1% of the total U.S. population in FY2011. The overall noncitizen proportion of the total prisoner population of 7.9% was up from 6.3% in 2001, with this change reflecting growth of the noncitizen population throughout federal and state prisons as well as local jails. Between 2001 and 2010, the noncitizen proportion of federal prisoners remained relatively stable at around 23% before increasing to 27% in 2011. The noncitizens proportion of state prisoners increased more slowly, growing from 4.3% of the total state-level incarcerated population in 2001 to 5.2% in 2011.