Congress has long supported efforts to identify, detain, and remove noncitizens who have been convicted of crimes in the United States. More generally, all unauthorized aliens within the United States are potentially subject to removal, and "interior enforcement" (i.e., alien removals originating from within the United States) is a basic element of immigration control. The criminal alien population, to which such enforcement efforts are targeted, is complex. It is defined in broad terms, challenging to quantify, and sufficiently large and diverse in its criminality that priorities have been established or modified to direct law enforcement efforts. This report discusses four programs operated by the Department of Homeland Security's (DHS) Immigration and Customs Enforcement (ICE) to identify and remove criminal and other removable aliens. The Criminal Alien Program (CAP) is an umbrella program for marshaling the agency's resources to identify and remove criminal and other removable aliens. CAP is guided by the Priority Enforcement Program (PEP ), which represents a set of enforcement priorities that describe which foreign nationals should be removed and in what priority order. PEP also comprises a data sharing infrastructure or "interoperability" between DHS and the Department of Justice that screens for immigration violations when individuals are booked into jails. ICE's §287(g) program allows 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 outside of controlled settings (i.e., administrative offices or custodial settings). PEP, its predecessor Secure Communities, and the §287(g) program have all contributed to DHS removing large numbers of aliens in the past decade. Yet, these programs also have been controversial. Because interoperability screens 100% of people passing through law enforcement jurisdictions, critics often charged ICE with removing many people who either committed minor crimes or who had no criminal record apart from unauthorized presence in the United States. Other critics charge that revisions to the set of enforcement priorities through PEP have since contributed to a sizable decline in the number of enforcement actions. The §287(g) program has raised concerns about the role of state and local law enforcement agencies in immigration enforcement. Inconsistent policies and practices among jurisdictions and allegations of racial profiling, among other issues, caused ICE to revise the program in FY2012 and allow certain §287(g) agreements with law enforcement agencies to expire. Since then, immigration enforcement advocates have questioned why ICE has curtailed the program's use. ICE has recently expressed interest in expanding it. For these and other reasons, Congress may be interested in measures of enforcement levels by program, the level of appropriations for different criminal alien programs, and the role of state and local law enforcement agencies in immigration enforcement. This report begins by defining the criminal alien population and quantifying a portion of it with the limited publically available data. Next it presents the most recent ICE enforcement priorities that emerged with the introduction of PEP in November 2014. The report then describes the major ICE immigration enforcement programs that target criminal aliens. The final section discusses selected policy considerations for programs targeting criminal aliens. This includes a discussion of current levels of enforcement and program appropriations, as well as state and local involvement in immigration enforcement. U.S. immigration law identifies certain crimes that make an alien ineligible for admission to the United States and/or subject to removal. 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 related terms.) Criminal aliens can be further distinguished by legal status, which in turn affects removability. Unauthorized aliens have made up roughly half of all U.S. noncitizens in recent years. The unauthorized alien population includes not only persons who entered without inspection or overstayed the terms of their temporary visas but also persons who have what some refer to as a "quasi-legal" status (e.g., temporary protected status, parole, deferred action) that affords them relief from immediate removal. Hence, not all unauthorized aliens living in the United States are subject to removal. Most unauthorized aliens, however, are removable; but few have been convicted of a crime and are classified as criminal aliens (unlawful presence in the United States itself is a civil violation, not a criminal offense). Three groups of criminal aliens can be distinguished. All criminal aliens include both unauthorized aliens, most of whom are potentially removable, and legal aliens who may or may not be removable depending on specific crimes committed. This population contains the set of criminal aliens who are removable on the basis of specific crimes committed. Criminal aliens who have been convicted of removable criminal offenses are subject to removal under the Immigration and Nationality Act (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. This population also includes aggravated felons. Criminal aliens who have been convicted of aggravated felonies are ineligible for most forms of relief from removal and are ineligible to be readmitted to the United States. As noted above, all three of these subpopulations—criminal aliens, removable criminal aliens, and aggravated felons—comprise an unknown mix of legally present noncitizens and unauthorized aliens. In addition, criminal aliens can be distinguished by whether they are currently incarcerated in federal and state prisons and local jails. Accordingly, the criminal alien population can include noncitizens convicted of crimes but who are not incarcerated, noncitizens convicted of crimes who are currently incarcerated, and noncitizens who have ever been convicted of crimes in the past, who have served their sentences, and who have been released from prison and reside in the United States. Having defined the criminal alien population, this report next turns to the issue of quantifying its size. This issue is complicated by the lack of data and published estimates on the criminal alien population. CRS is not aware of any authoritative current estimate for the total criminal alien population . DHS has produced its own estimates of the removable criminal alien population , a subgroup of the total criminal alien population. In 2013, ICE published estimates indicating that approximately 900,000 aliens were arrested for crimes every year; that approximately 550,000 criminal aliens convicted of crimes exited law enforcement custody every year; and that 1.9 million removable criminal aliens currently resided in the United States. This CRS report presents data on authoritative published measures that quantify one portion of the criminal alien population—the noncitizen population incarcerated in federal and state prisons and local jails. Incarcerated noncitizens represent a subset of criminal aliens, but given the paucity of data on the criminal alien population, it represents one consistent annual measure that can be compared both across time and relative to the total U.S. incarcerated population. This section presents incarceration data from 2002 through 2013. It then discusses the quality and limitations of publically available data pertaining to the criminal alien population. Table 1 presents CRS tabulations of the total year-end U.S. citizen and noncitizen populations in federal and state prisons and local jails publicly reported by the Bureau of Justice Statistics for 2002 through 2013 (the most recent year for which complete data are available). This population includes an unknown portion of persons who fall into each of the three criminal alien populations discussed above: total criminal aliens, removable criminal aliens, and aggravated felons. This population also includes both persons lawfully present in the United States and unauthorized aliens. Federal prison data come from annual published BJS reports that compile year-end data collected from the Bureau of Prisons (BOP) under the auspices of BJS's National Prisoner Statistics (NPS) program. State prison data also come from annually published BJS reports that compile mid-year and year-end data collected by the NPS program from the correctional departments of all 50 states. Local jail data come from an annual BJS survey, the Annual Survey of Jails , from which it creates national-level estimates of the number of inmates incarcerated in local jails. In addition, it conducts a complete count of the local jail population every five years through its Census of Jails . Together, these two local jail counts provide a consistent annual series of the total number of persons incarcerated in local jails. Table 1 indicates that as of December 31, 2013, a total of 2,308,158 prisoners (U.S. citizens and noncitizens) were incarcerated: 215,866 in federal prisons; 1,361,084 in state prisons; and 731,208 in local jails. The 142,463 noncitizens reported in federal and state prisons and local jails at year-end 2013 represented 6.2% of the total incarcerated population. Noncitizen inmates comprised 23.5% of the total federal prison population, 3.5% of the total state prison population, and 6.0% of the total local jail population in 2013. The large noncitizen proportion in federal prisons occurs for two reasons. First, immigration offenses, which are prosecuted at the federal level, represent a disproportionate share of federal offenses for which convicted individuals can be incarcerated in federal prisons. Second, data on noncitizens incarcerated in federal prisons are tabulated more accurately and completely than data on those incarcerated in state prisons and local jails, because federal data are collected consistently by one administrative unit using a single definition of noncitizen.