While the power to prescribe rules as to which aliens may enter the United States and which aliens may be removed resides solely with the federal government, the impact of alien migration—whether lawful or unlawful—is arguably felt most directly in the communities where aliens settle. State and local responses to unlawfully present aliens within their jurisdictions have varied considerably, particularly as to the role that state and local police should play in enforcing federal immigration law. At one end of the spectrum, some states and localities have actively sought to deter unlawfully present aliens from settling within their jurisdictions, including by assisting federal immigration authorities in identifying and apprehending such aliens for purposes of removal. In some cases, this has involved state and local participation in federally coordinated immigration enforcement programs. In recent years, some states and localities have attempted to play an even greater role in the area of immigration enforcement, in many cases due to perceptions that federal efforts have been inadequate. Some have adopted measures that criminally sanction conduct believed to facilitate the presence of unlawfully present aliens, and have also instructed police to actively work to detect such aliens as part of their regular duties. The adoption of such measures has waned considerably, however, in the aftermath of the 2012 Supreme Court ruling in Arizona v. United States , where the Court held that many of the provisions of one such enactment, Arizona's S.B. 1070, were facially preempted by federal immigration law. At the other end of the spectrum, some states and localities have been unwilling to assist the federal government's enforcement of measures that distinguish between those residents with legal immigration status and those who lack authorization under federal law to be present in the United States. In some instances, these jurisdictions have adopted measures that seek to thwart federal efforts to identify and apprehend unlawfully present aliens within the state or locality's jurisdiction. While state and local restrictions on cooperation in federal immigration enforcement efforts have existed for decades, there has been an upswing in the adoption of these measures in recent years. Moreover, the nature of these restrictions has evolved over time, particularly in response to recent federal immigration enforcement initiatives like Secure Communities (subsequently replaced by the Priority Enforcement Program (PEP), as discussed below), which enable federal authorities to more easily identify potentially removable aliens in state or local custody. This report discusses legal issues related to state and local measures limiting law enforcement cooperation with federal immigration authorities. It begins by providing a brief overview of constitutional principles informing the relationship between federal immigration authorities and state and local jurisdictions. The report then discusses various types of measures adopted or considered by states and localities to limit their participation with federal immigration enforcement efforts, including (1) limiting police investigations into the immigration status of persons with whom they come in contact; (2) declining to honor federal immigration authorities' requests that certain aliens be held until those authorities may assume custody; (3) shielding certain unlawfully present aliens from detection by federal immigration authorities; and (4) amending or applying state criminal laws so as to reduce or eliminate the immigration consequences that might result from an alien's criminal conviction. Pursuant to its "broad, undoubted power over the subject of immigration and the status of aliens," the federal government has established an "extensive and complex" set of rules governing the admission and removal of aliens, along with conditions for aliens' continued presence within the United States. These rules are primarily contained in the Immigration and Nationality Act of 1952, as amended (INA). The INA supplements these rules through an enforcement regime that contains criminal and civil provisions, which sometimes sanction similar conduct. The courts have consistently recognized that the removal of aliens from the United States for violating the terms of their admission or continued presence in the country is a civil action, rather than a criminal sanction, because the main purpose is not to punish wrongdoing but to end a continuing violation of the nation's immigration laws. While the federal government's authority over immigration is well established, the Supreme Court has recognized that not "every state enactment which in any way deals with aliens is a regulation of immigration and thus per se preempted" by the federal government's exclusive power over immigration. The Tenth Amendment provides that powers "not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people." Among the powers reserved to the states are traditional "police powers" concerning the promotion and regulation of safety, health, welfare, and economic activity within the state's jurisdiction. Pursuant to the exercise of these powers, states and municipalities have frequently enacted measures which, directly or indirectly, address aliens residing in their communities. The exercise of state police powers may be circumscribed by lawful assertions of federal authority. The Supremacy Clause of the Constitution establishes that federal law, treaties, and the Constitution itself are "the supreme Law of the Land." Accordingly, states and localities may be precluded from taking actions that are "preempted" by federal law, even if such actions are otherwise valid exercises of their police powers. An act of Congress may preempt state or local action in one of three ways: (1) the statute expressly indicates its preemptive intent (express preemption); (2) Congress intended to wholly occupy the regulatory field, thereby implicitly precluding supplemental action by a state or local government in that area (field preemption); or (3) state or local action conflicts with or otherwise frustrates the purpose of the federal scheme (conflict preemption). Congressional intent is paramount in the analysis as to whether federal law preempts state or local activity. However, courts' preemption analysis generally begins with the "assumption that the historic police powers of the States were not to be superseded by the Federal Act unless that was the clear and manifest purpose of Congress." Acting pursuant to its authority over immigration, the federal government has expressly or impliedly preempted a range of state and local activities that potentially undermine or conflict with federal immigration enforcement policies. Of particular relevance to this report, pursuant to §434 of the Personal Responsibility and Work Opportunity Reconciliation Act of 1996 (PRWORA, P.L. 104-193 ) and §642 of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA, P.L. 104-208 , Div. C), Congress has preempted state or municipal measures that bar the exchange of information relating to persons' immigration status with federal, state, or local authorities. While the federal government's power to preempt activity in the area of immigration is extensive, there are constitutional limits to its power to influence state and local activity. Notably, it may not directly "commandeer" state or local governments into the service of federal immigration authorities. The anti-commandeering doctrine was most prominently defined by the Supreme Court in the cases of New York v. United States and Printz v. United States . The decisions were premised on the view that under the federalist system, the states are understood to be sovereign entities distinct from the federal government, and Congress cannot muddy this distinction by commandeering the state political branches to perform functions on the federal government's behalf. In New York , the Supreme Court ruled that the Tenth Amendment barred the federal government from directly compelling state legislatures to "enact and enforce a federal regulatory program" related to the disposal of low-level radioactive waste. In Printz, the Court considered a Tenth Amendment challenge to an interim requirement under federal law that directed state and local police to conduct background checks on prospective handgun purchases. The Court ruled that the constitutional prohibition on the federal government commandeering states to administer a federal regulatory program was "categorical" in nature and, in addition to barring the commandeering of a state's legislature, also prevented Congress from "conscripting the State's officers directly." However, not every requirement imposed by the federal government upon sub-federal government entities and officials necessarily violates the anti-commandeering principles identified in Printz and New York . A number of federal statutes provide that certain information collected by state entities must be reported to federal agencies. The Court in Printz expressly declined to consider whether these requirements were constitutionally impermissible. Moreover, the Court distinguished reporting requirements from the case before it, which involved "the forced participation of the States ... in the actual administration of a federal program." Federal measures that impose direct requirements on state or municipal authorities appear most likely to withstand an anti-commandeering challenge if they (1) are not directed at a state's regulation of the activities of private parties; and (2) apply to the activities of private parties as well as government actors. In Reno v. Condon , the Supreme Court unanimously rejected a Tenth Amendment challenge to the Driver's Privacy Protection Act (DPPA), which restricted states from disclosing or sharing a driver's personal information without the driver's consent, and also required the disclosure of some collected information. The Court distinguished the DPPA from the federal enactments struck down in New York and Printz , because the DPPA sought to regulate states "as owners of databases" and did not "require the States in their sovereign capacity to regulate their own citizens ... [or] enact any laws or regulations ... [or] require state officials to assist in the enforcement of federal statutes regulating private individuals." The Court also viewed it as significant that the DPPA's requirements relating to information-sharing covered private entities as well as state governments, though it declined to definitively rule upon whether or not the federal government may directly regulate state conduct only through laws of general applicability. While the federal government may be constitutionally barred from conscripting state authorities into assisting in the administration of a federal program, other means may be available to influence states to adopt favored policies. For example, when Congress acts in an area in which it may preempt state activity in its entirety, it might impose "preconditions to continued state regulation" in the otherwise preempted field. Congress may also permissibly condition the receipt of federal funds on state compliance with federal policy preferences. Conditioning the receipt of federal funding is generally permissible so long as the conditions "bear some relationship to the purpose of the federal spending," and the conditioned funds are not so substantial that the inducement to comply with federal preferences is "so coercive as to pass the point at which 'pressure turns into compulsion.'" Several states and municipalities have considered or adopted measures intended to limit their participation in federal immigration enforcement efforts. These limitations take several forms. Some states and localities have sought to restrict police cooperation with federal immigration authorities' efforts to apprehend removable aliens. Other measures have gone further, and attempted to shield certain aliens from detection by federal authorities. Still other measures have sought to ensure that state convictions for certain criminal offenses do not carry immigration consequences for a convicted alien. The following sections discuss some of the notable state and local restrictions upon law enforcement activity in the field of immigration enforcement, including the relationship between these restrictions and current federal law. A number of states and municipalities have adopted formal or informal policies which prohibit or substantially restrict police cooperation with federal immigration enforcement efforts. Entities that have adopted such policies are sometimes referred to as "sanctuary" jurisdictions, though there is not necessarily a consensus as to the meaning of this term. In some instances, jurisdictions have self-identified as "sanctuary" sites. In other cases, there might be disagreement regarding the accuracy of such a designation, particularly if state or local law enforcement cooperates with federal immigration authorities in some areas but not others. Any reference by this report to a policy of a particular jurisdiction is intended only to provide an illustrative example of the type of measure occasionally referenced in discussions of "sanctuary" policies. These references should not be taken to indicate CRS is of the view that a particular jurisdiction is a "sanctuary" for unlawfully present aliens. While state or local measures limiting police participation in immigration enforcement existed beforehand, many of the recent "sanctuary"-type initiatives trace their lineage back to actions by U.S. churches that provided refuge to unauthorized Central American aliens fleeing civil unrest in the 1980s. A number of states and municipalities issued declarations in support of these churches' actions. Others went further and enacted more substantive measures intended to limit police involvement in federal immigration enforcement activities. These have included, among other things, restricting state and local police from arresting persons for immigration violations; limiting the sharing of immigration-related information with federal authorities, and barring police from questioning a person about his or her immigration status. As previously noted, violations of federal immigration law may be criminal or civil in nature, with alien removal understood to be a civil proceeding. Some immigration-related conduct potentially constitutes a removable offense and may also be subject to criminal sanction. For example, an alien who knowingly enters the United States without authorization is not only potentially subject to removal, but could also be charged with a criminal offense relating to unlawful entry. On the other hand, some violations of the INA are exclusively criminal or civil in nature. Most notably, an alien's unauthorized immigration status makes him or her removable, but absent additional factors (e.g., having reentered the United States after being formally removed), unlawful presence does not constitute a criminal offense. Some jurisdictions have adopted formal or informal measures that restrict or bar police officers from making arrests for violations of federal immigration law. In some cases, these restrictions prohibit police from making arrests for "civil" violations of federal immigration law, such as unlawful presence. In other instances, these restrictions are crafted more broadly and may also restrict arrests for some criminal violations of federal immigration law. State or local restrictions on police authority to arrest persons for federal immigration law violations do not appear to raise significant legal issues. While the INA expressly allows state and local law enforcement to directly engage in specified immigration enforcement activities within the parameters established by the applicable INA provision, nothing in the INA directly compels such participation (and indeed, any requirement would raise significant anti-commandeering issues under the Tenth Amendment). Moreover, following the Supreme Court's decision in Arizona v. United States , it appears that states and localities are generally preempted from making arrests for civil violations of the INA in the absence of either specific federal statutory authorization or the "request, approval, or instruction from the Federal Government." Over the years, some states and localities restricted government agencies or employees from sharing information with federal immigration authorities, primarily to prevent federal authorities from using such information to identify and apprehend unlawfully present aliens for removal. Some of these restrictions have existed for decades, while others are of more recent vintage. In 1996, Congress sought to end these restrictions on information-sharing through provisions contained in the Personal Responsibility and Work Opportunity Reconciliation Act (PRWORA) and Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA). Neither PRWORA nor IIRIRA require state or local government entities to share immigration-related information with federal authorities. Instead, these provisions bar any restrictions that prevent state or local government entities or officials from voluntarily communicating with federal immigration authorities regarding a person's immigration status. PRWORA §434 bars state and local governments from imposing any prohibition or restriction on a state or local government entity that prevents it from sending or receiving information, to or from federal immigration authorities, regarding the "immigration status" of an individual. IIRIRA §642 is broader and more detailed in scope. It bars any restriction on a federal, state, or local governmental entity or official's ability to send or receive information regarding "immigration or citizenship status" to or from federal immigration authorities. It further provides that no person or agency may prohibit a federal, state, or local government entity from (1) sending information regarding immigration status to, or requesting information from, federal immigration authorities; (2) maintaining information regarding immigration status; or (3) exchanging such information with any other federal, state, or local government entity. Shortly after these measures were enacted, the City of New York, which had in effect a policy that limited the sharing of information with federal immigration authorities, brought suit challenging the constitutionality of PRWORA §434 and IIRIRA §642. Among other things, the city alleged that the provisions facially violate the Tenth Amendment, because they bar states and localities from controlling the degree to which their officials may cooperate with federal immigration authorities. A federal district court dismissed this claim in City of New York v. United States , and a three-judge panel of the U.S. Court of Appeals for the Second Circuit affirmed this ruling in a 1999 decision. The circuit court observed that unlike the statutes struck down in New York and Printz , the information-sharing provisions in PRWORA and IIRIRA did not directly compel state authorities to administer and enforce a federal regulatory program. Instead, these provisions protected "the voluntary exchange" of immigration information between federal and state authorities. According to the circuit court, "informed, extensive, and cooperative interaction of a voluntary nature" between states and the federal authorities is an integral feature of the American system of dual sovereignty, and the information-sharing provisions of PRWORA and IIRIRA were consistent with these principles. The Second Circuit raised the possibility that the federal government might, in some circumstances, impermissibly intrude upon state and local entities' authority to control information in their possession. However, the court found that the City of New York had not provided evidence that this was such an instance, as the affected city policy only limited the sharing of a particular type of information (i.e., a person's immigration status) with a particular federal agency (i.e., the agency responsible for enforcing federal immigration law). According to the circuit court, the Tenth Amendment does not provide states and local entities with the "untrammeled right to forbid all voluntary cooperation by state or local officials with particular federal programs." The court therefore rejected the city's constitutional challenge to the information-sharing provisions of PRWORA and IIRIRA, finding that they did not facially violate the Tenth Amendment. The City of New York sought to appeal the decision to the Supreme Court, but its petition for certiorari was denied. A few months later, however, the Court issued its decision in Reno v. Condon , discussed in more detail earlier in this report, where it found that another federal statute that regulated the dissemination of information collected by state authorities did not violate the Tenth Amendment. While it might be argued that the Condon decision provides support for the constitutional validity of PRWORA §434 and IIRIRA §642, no court appears to have assessed the implications of the Condon decision upon these measures. Since the Second Circuit's ruling, it appears that there have been no judicial rulings that have questioned the validity of the information-sharing provisions in PRWORA and IIRIRA. Although some state and local measures that purport to limit officials from sharing immigration-related information with federal immigration authorities remain in effect, any attempt by the state or locality to enforce these restrictions on information-sharing could be challenged on preemption grounds. Most traditional "sanctuary" policies place restrictions upon police inquiries or investigations into a person's immigration status. Some policies provide, for example, that police should not question a person about his or her immigration status except as part of a criminal investigation. Other policies might provide more specific restrictions that focus upon law enforcement's questioning of crime victims and witnesses. Still other policies more broadly limit state or local officials from gathering information about persons' immigration status. Although PRWORA §434 and IIRIRA §642 prevent state or local restrictions on sharing information about a person's immigration status with federal immigration authorities, these provisions do not require state or local police to actually collect such information. Restricting the authority of police to question a person about his or her immigration status helps ensure that law enforcement lacks any information that could be shared with federal immigration authorities. It could be argued that, even though state or local restrictions on police questioning of persons regarding their immigration status is not expressly preempted by federal statute, these measures are nonetheless impliedly preempted by the information-sharing provisions of IIRIRA and PRWORA. However, this argument was rejected by a California state appellate court in the context of a legal challenge to the Los Angeles Police Department's restrictions on investigations into persons' immigration status. The federal courts have not directly considered this issue. In Arizona v. United States , the Supreme Court found that a provision of an Arizona statute, which required police to contact federal authorities to verify the immigration status of certain stopped individuals, was not facially preempted. In reaching this conclusion, the Court did not suggest that federal law might preempt states or localities from restricting the circumstances in which police might question individuals about their immigration status. Indeed, given that the Arizona Court held that state and local police were largely preempted from making arrests for immigration status violations, it seems unlikely that a federal court would find that state or local measures that limited police questioning of persons about their immigration status would be viewed as preempted by the INA. An immigration detainer is a document by which U.S. Immigration and Customs Enforcement (ICE) advises other law enforcement agencies of its interest in individual aliens whom those agencies are currently holding in relation to criminal violations. ICE and its predecessor, the Immigration and Naturalization Service (INS), have used detainers as one means of obtaining custody of aliens for purposes of removal proceedings since at least 1950. However, ICE's implementation of the Secure Communities program in the period between 2008 and 2014 raised numerous questions about detainer s. This program relied upon information sharing between various levels and agencies of government to identify potentially removable aliens. Detainers were then issued for some of these aliens. DHS emphasized that it prioritized "criminal aliens," those who posed a threat to public safety, and repeat immigration violators for removal through Secure Communities, and the former Director of ICE further instructed that, among "criminal aliens," the focus was to be upon those convicted of "aggravated felonies," as defined in the INA; those convicted of other felonies; and those convicted of three or more misdemeanors. However, there were reports of detainers issued for persons who were not convicted of any offense, or whose sole offense was a misdemeanor. As a result of these and related reports, several jurisdictions adopted policies of declining immigration detainer requests for at least some aliens. In many cases, these jurisdictions will honor immigration detainers for aliens who are being held for felony crimes or who are otherwise believed to pose a threat to the community, but will decline to honor immigration detainers issued for other aliens (e.g., those who have committed non-violent misdemeanor offenses). In other cases, however, the measure appears to sweep more broadly, for example, barring officials from honoring immigration detainers absent a "written agreement with the federal government by which all costs incurred by [the jurisdiction] in complying with the ICE detainer shall be reimbursed." The detainer policy may also restrict other aspects of the jurisdiction's response to federal efforts to identify and/or obtain custody of aliens for purposes of removal. Although criticized by some for impeding federal immigration enforcement efforts, state or local policies of declining to honor at least some immigration detainers would appear to be permissible under federal law. Nothing in the INA purports to require that states and localities honor immigration detainers. The detainer regulations do use language that some have opined means that holds pursuant to detainers, at least, are required, stating that "[u]pon a determination by [ICE] to issue a detainer for an alien not otherwise detained by a criminal justice agency, such agency shall maintain custody of the alien for a period [generally] not to exceed 48 hours." However, the caption of the section containing this language refers to "temporary detention at [ICE] request," and the Executive has taken the position that the regulation requires that any period of time that a state or locality holds an alien pursuant to a detainer generally be limited to 48 hours, and not as requiring states or localities to hold aliens pursuant to detainers. Several federal courts have similarly characterized detainers as requests or otherwise agreed with the Executive's interpretation here, in part, because any purported requirement that states or localities hold aliens for ICE could be seen to run afoul of the anti-commandeering principles of the Tenth Amendment. For example, in its 2014 decision in Galarza v. Szalczyk , a majority of a three-judge panel of the U.S. Court of Appeals for the Third Circuit noted if states and localities were required to detain aliens for ICE, they would have to "expend funds and resources to effectuate a federal regulatory scheme," something found to be impermissible in prior Supreme Court decisions regarding commandeering. The Galarza majority also found that such a requirement would be "exactly the type of command that has historically disrupted our system of federalism" by obscuring which level of government is accountable for particular policies. Note, however, that DHS's recent discontinuation of the Secure Communities program and its establishment of a new Priority Enforcement Program (PEP) could allay certain of the concerns that prompted some jurisdictions to adopt policies of declining immigration detainers. PEP is like Secure Communities in that it "will continue to rely on fingerprint-based biometric data submitted during bookings by state and local law enforcement agencies to the Federal Bureau of Investigation for criminal background checks." However, with PEP, detainers are to be issued only for aliens who have been convicted of (rather than just arrested for ) certain offenses that are among ICE's priorities for civil immigration enforcement. Further, with PEP, detainers are generally to be used only to request that state and local law enforcement officials notify ICE prior to the alien's release or transfer to another institution, not to request that state and local officials detain aliens beyond the point when they otherwise would be released for the state or local offense so that ICE may assume custody. Any detainers issued to request detention (as opposed to notification) would generally have to specify that (1) the alien is subject to a final order of removal, or (2) "there is other sufficient probable cause to find that the person is a removable alien." One way in which the INA regulates immigration is by establishing the conditions under which aliens may enter and remain in the United States. Along with these conditions, it imposes sanctions to deter unlawful entry or presence in the United States. In particular, INA §274 imposes criminal penalties upon those who would transport unlawfully present aliens to or within the United States, or shield such aliens from detection by federal immigration authorities. Some activities by state or local governments or private parties have the potential to run afoul of these "anti-transportation" and "anti-harboring" provisions because the traditional conception of sanctuary entails providing material assistance—most notably, a place of refuge—to those in need. Indeed, several individual participants in the sanctuary movement in the 1980s were convicted of violating §274 because of their efforts to move unlawfully present aliens within the United States and ensure they were not detected by federal immigration officials. At least as a general rule, state or local measures that restrict involvement with federal immigration enforcement efforts typically would not appear to implicate the anti-harboring provisions of federal immigration law. However, a state or local policy under which government officials affirmatively took steps to prevent the federal government from removing a particular alien might raise more serious concerns, particularly if the officials within the jurisdiction know (or have reason to know) that specific individuals are unlawfully present and play a "key role" in furthering the aliens' unlawful presence (in the case of transportation), or "substantially facilitate" their continued presence in the United States (in the case of harboring). Such policies or practices would appear to be most likely to develop vis-à-vis specific segments of the unlawfully present alien population which are seen as particularly vulnerable to mistreatment, such as juveniles. For example, beginning in 2008, San Francisco's practice of flying unlawfully present aliens in the juvenile justice system who do not have a parent or guardian in the United States back to their home countries, or placing them in group homes in southern California, was subject to a federal grand jury investigation of potential violations of §274. Whether such policies did indeed run afoul of the anti-harboring provisions in the INA was not definitively determined, as San Francisco reportedly abandoned its policy and federal criminal charges were not pursued. An alien's conviction for a crime under state law may also carry consequences for the alien under federal immigration law. A criminal conviction may, depending on the circumstances, subject the alien to removal under the INA, disqualify the alien from obtaining relief from removal (e.g., asylum) or an immigration benefit, and potentially result in the alien being barred from re-entering the United States once removed. Because states are responsible for the vast majority of criminal prosecutions in the United States, they have a substantial influence on the size and make-up of the alien population who are removable on criminal grounds. Some state authorities have sought to reduce or eliminate the immigration consequences of certain criminal convictions. In many instances, whether an alien's criminal activity makes him removable under the INA depends both upon the nature of the criminal offense for which the alien was convicted and the sentence that was imposed. A few states have lowered the maximum available sentence for certain offenses to 364 days' imprisonment, so that aliens will not be subject to removal under those grounds of the INA requiring the alien to have been sentenced to at least a year's imprisonment. State courts may consider the immigration consequences of a criminal conviction or sentence during the course of criminal proceedings against a foreign national. In some situations, state courts (possibly with the recommendation of the state prosecutor) might resolve a criminal case in a manner that avoids triggering immigration consequences for the criminal defendant. For example, the court might authorize the parties to enter an agreement that resolves the case in a manner that does not constitute a "conviction" for INA purposes. If an alien is convicted of a crime that constitutes a removable offense under the INA only when the alien is sentenced to a specified length of imprisonment, the court might opt to sentence the alien to a lesser term that does not result in removability. A court might also choose to modify an already-imposed sentence so that it no longer has immigration consequences. Federal legislative proposals have occasionally been considered that would negate the effects of some of these actions by state courts, but none of these bills have been enacted.

Summary: While the power to prescribe rules as to which aliens may enter the United States and which aliens may be removed resides solely with the federal government, the impact of alien migration—whether lawful or unlawful—is arguably felt most directly in the communities where aliens settle. State and local responses to unlawfully present aliens within their jurisdictions have varied considerably, particularly as to the role that state and local police should play in enforcing federal immigration law. Some states, cities, and other municipalities have sought to play an active role in immigration enforcement efforts. However, others have been unwilling to assist the federal government in enforcing measures that distinguish between residents with legal immigration status and those who lack authorization under federal law to be present in the United States. In some circumstances, these jurisdictions have actively opposed federal immigration authorities' efforts to identity and remove certain unlawfully present aliens within their jurisdictions. Although state and local restrictions on cooperation with federal immigration enforcement efforts have existed for decades, there has reportedly been an upswing in the adoption of these measures in recent years. Moreover, the nature of these restrictions has evolved over time, particularly in response to recent federal immigration enforcement initiatives like Secure Communities (subsequently replaced by the Priority Enforcement Program), which enable federal authorities to more easily identify removable aliens in state or local custody. Entities that have adopted such policies are sometimes referred to as "sanctuary" jurisdictions, though there is not necessarily a consensus as to the meaning of this term or its application to a particular state or locality. Recent reports that an alien who shot and killed a woman, after being released by San Francisco authorities who had declined to honor an immigration detainer issued by federal authorities, have brought increased attention to state and local practices of declining to honor such detainers, as well as "sanctuary" policies more generally. This report discusses legal issues related to state and local measures that limit law enforcement cooperation with federal immigration authorities. The report begins by providing a brief overview of the constitutional principles informing the relationship between federal immigration authorities and state and local jurisdictions, including the federal government's power to preempt state and local activities under the Supremacy Clause, and the Tenth Amendment's proscription against Congress directly "commandeering" the states to administer a federally enacted regulatory scheme. The report then discusses various types of measures adopted or considered by states and localities to limit their participation in federal immigration enforcement efforts, including (1) limiting police investigations into the immigration status of persons with whom they come in contact; (2) declining to honor federal immigration authorities' requests that certain aliens be held until those authorities may assume custody; (3) shielding certain unlawfully present aliens from detection by federal immigration authorities; and (4) amending or applying state criminal laws so as to reduce or eliminate the immigration consequences that might result from an alien's criminal conviction. For discussion of legal issues raised by states and localities seeking to play an active role in enforcing federal immigration law, see CRS Report R41423, Authority of State and Local Police to Enforce Federal Immigration Law, by [author name scrubbed] and [author name scrubbed].