The term prosecutorial discretion is commonly used to describe the "wide latitude" that prosecutors have in determining when, whom, how, and even whether to prosecute apparent violations of the law. The Immigration and Naturalization Service (INS) and, later, the Department of Homeland Security (DHS) and its components have historically described themselves as exercising prosecutorial discretion in the enforcement of federal immigration law, which is largely contained in the Immigration and Nationality Act of 1952 (INA), as amended . Some commentators have recently challenged this characterization on the grounds that DHS enforces primarily civil violations, and some of its components cannot be said to engage in "law enforcement," as that term is conventionally understood. However, even agencies that do not prosecute or engage in law enforcement have been recognized as having discretion (sometimes referred to as enforcement discretion ) in determining whether to enforce particular violations, and immigration officers appear to have exercised such discretion in individual cases and on a categorical basis for decades. For example, the Kennedy Administration granted extended voluntary departure to persons from Cuba in 1960, allowing many otherwise deportable Cuban nationals to remain in the United States for an extended period, while the George W. Bush Administration temporarily suspended employer sanctions on entities that employed unauthorized aliens in areas affected by Hurricane Katrina. The scope of prosecutorial discretion in immigration enforcement has recently been of interest to Congress and the public due to certain initiatives of the Obama Administration. In 2011, John Morton, then Director of U.S. Customs and Immigration Enforcement (ICE), issued two memoranda addressing prosecutorial discretion, one of which identified ICE's priorities for the apprehension, detention, and removal of aliens, and the other of which discussed how ICE personnel may exercise prosecutorial discretion consistent with ICE's enforcement priorities. Subsequently, in June 2012, then Secretary of Homeland Security Janet Napolitano issued a memorandum "setting forth how, in the exercise of [its] prosecutorial discretion, the Department ... should enforce the Nation's immigration laws against certain young people who were brought to this country as children and know only this country as home." As implemented, this initiative has come to be known as Deferred Action for Childhood Arrivals (DACA). Most recently, ICE has directed its personnel to exercise discretion in "ensur[ing] that the agency's immigration enforcement activities do not unnecessarily disrupt the parental rights of both alien parents or legal guardians of minor children. " These initiatives have been challenged by some Members of Congress and commentators on the grounds that they are tantamount to "amnesty" for unauthorized aliens and are contrary to the President's constitutional responsibility to "take Care" that the laws be enforced. In particular, some Members have suggested that DACA exceeds the President's authority because "it was issued after Congress specifically rejected legislation"—the Development, Relief, and Education for Alien Minors (DREAM) Act—"embodying that policy." In addition, several ICE agents and the State of Mississippi filed suit in federal district court for the Northern District of Texas alleging that the DACA initiative violates certain statutory requirements and impinges upon Congress's legislative powers, among other things. This report begins by discussing the sources of federal power to regulate immigration and, particularly, the allocation of power between Congress and the President in this area. It next addresses the constitutional and other foundations for the doctrine of prosecutorial discretion, as well as the potential ways in which prosecutorial discretion may be exercised in the immigration context. It concludes by addressing potential constitutional, statutory, and administrative constraints upon the exercise of prosecutorial discretion. The report does not address other aspects of discretion in immigration law, such as the discretion exercised by immigration officers in granting benefits (e.g., asylum), or by immigration judges in non-enforcement contexts (e.g., cancellation of removal). The Constitution does not directly address the sources of federal power to regulate which non-U.S. nationals (aliens) may enter and remain in the United States, or to establish the conditions of their continued presence within the country. However, several of the enumerated powers of the federal government have been construed as authorizing such regulation. The powers to establish a uniform rule of naturalization and regulate commerce are arguably the most commonly cited provisions, particularly in recent years. Various authorities related to foreign affairs have also been routinely cited as providing support for particular enactments and activities in the field of immigration. In addition, in some cases, the Supreme Court has suggested that federal regulation of immigration is grounded in the federal government's "inherent power as a sovereign to control and conduct foreign relations." Many, although not all, of these powers belong exclusively to Congress, and courts and commentators have sometimes used language which implies that Congress is preeminent in the field of immigration. For example, it has frequently been said that Congress has "plenary power" over immigration, and that "over no conceivable subject is the legislative power of Congress more complete than it is over" immigration. In some cases, courts have even suggested that the executive branch's authority over immigration arises from a delegation of congressional power, as is the case with other Article I powers, although Article I does not give Congress clear supremacy over immigration, as previously noted. In Sale v. Haitian Centers Council, Inc. , for example, the Supreme Court rejected a challenge which alleged that the executive branch's procedures for screening Haitian migrants at sea, without allowing them to disembark in the United States, did not comply with statutory and treaty-based protections that enable aliens to apply for refugee status and avoid repatriation. The Court did so, in part, on the grounds that "[t]he laws that the Coast Guard is engaged in enforcing when it takes to the seas under orders to prevent aliens from illegally crossing our borders are laws whose administration has been assigned to the Attorney General by Congress." Similarly, in other cases, the Court has described Congress's power to exclude aliens from the United States, or prescribe the terms and conditions upon which they may enter, as being "enforced exclusively through executive officers," or opined that executive branch officials "exercise[] delegated legislative power" in taking specific actions. Few courts or commentators have, however, directly addressed the separation of powers between Congress and the President in the field of immigration, and in some cases, the Court has also suggested that the executive branch shares plenary power over immigration with Congress as one of the "political branches." While some such cases could potentially be construed as referring to powers delegated to the executive branch by Congress, in other cases, the President has been expressly said to have inherent authority over at least some immigration-related matters. For example, in United States ex rel. Knauff v. Shaughnessy , the Court upheld the executive branch's decision to exclude a German "war bride," in part, on the grounds that The exclusion of aliens is a fundamental act of sovereignty. The right to do so stems not alone from legislative power but is inherent in the executive power to control the foreign affairs of the nation.... When Congress prescribes a procedure concerning the admissibility of aliens, it is not dealing alone with a legislative power. It is implementing an inherent executive power. Similarly, in Hampton v. Mow Sung Wong , the Court indicated that certain rules adopted by the U.S. Civil Service Commission barring resident aliens from employment in the federal civil service impermissibly deprived these aliens of due process of law, but that such rules would be permissible if they "were expressly mandated by the Congress or the President." Here, Congress had delegated authority to the President to prescribe regulations for the admission of individuals to the civil service. Accordingly, it is possible that when the Hampton Court referred to the President's power to limit alien eligibility for federal employment, it intended to refer only to the power which had been conferred to him by Congress. On the other hand, the Court's discussion of the interests of the President that might be sufficient to justify the exclusion of noncitizens from the civil service focused upon the President's power to negotiate treaties, suggesting recognition of some independent constitutional basis for executive branch activity in the field of immigration. The possibility of independent executive branch authority over immigration is significant in that any such authority could potentially help justify certain actions taken by the executive branch (although actions taken in reliance on such authority could also potentially raise issues if they were arguably within Congress's purview). However, the executive branch's authority to exercise prosecutorial or enforcement discretion has traditionally been understood to arise from the Constitution, as discussed below. Courts have historically not required that the executive branch have specific statutory authorization for particular exercises of prosecutorial discretion. Thus, immigration officials would not necessarily be precluded from granting deferred action, or taking certain other actions that could permit otherwise removable aliens to remain in the United States, just because federal immigration statutes do not expressly authorize such actions. On the other hand, Section 103(a)(3) of the INA authorizes the Secretary of Homeland Security to "perform such other acts as he deems necessary for carrying out his authority under the provisions of this chapter," and has been construed by some as "commit[ting] enforcement of the INA to [the Secretary's] discretion." The federal government recently noted its discretion under Section 103(a)(3) in seeking dismissal of a lawsuit challenging the DACA initiative. The Secretary's authority under Section 103(a)(3) of the INA is, however, an authority granted to the executive branch by Congress and, as such, is distinguishable from the President's constitutional authority to "take Care" that the laws be enforced. Congress could, for example, potentially limit the discretion granted to the Secretary by Section 103(a)(3) of the INA, including by prohibiting particular exercises of discretion. In contrast, Congress probably could not directly limit the President's authority under the Constitution to "take Care" that the laws be enforced.