In precedent stretching back to the Chinese Exclusion Case of 1889, the Supreme Court has held that Congress possesses "plenary power" to regulate immigration. This power, according to the Court, is the most complete that Congress possesses. It allows Congress to make laws concerning aliens that would be unconstitutional if applied to citizens. And while the immigration power has proven less than absolute when directed at aliens already physically present within the United States, the Supreme Court has interpreted the power to apply with most force to the admission and exclusion of nonresident aliens abroad. The Court has upheld or shown approval of laws excluding aliens on the basis of ethnicity, gender and legitimacy, and political belief. Outside of the immigration context, in contrast, laws that discriminate on such bases are almost always struck down as unconstitutional. To date, the only established limitation on Congress's power to exclude aliens concerns lawful permanent residents (LPRs): they, unlike nonresident aliens, generally cannot be denied entry without a fair hearing as to their admissibility. The plenary power doctrine has long drawn scholarly criticism. Some legal commentators contend that the doctrine lacks a coherent rationale, and that it is an anachronism belonging to an earlier era of constitutional law predating the development of modern individual rights jurisprudence. More than 125 years after its initial recognition of the plenary power doctrine, however, the Supreme Court has continued to rely on it in immigration cases. Some commentators have argued that the Court is in the process of narrowing the parameters of the doctrine's applicability, although they find support for this argument mainly in cases outside of the exclusion context. The Constitution does not mention immigration. It does not expressly confer upon any of the three branches of government the power to control how citizens of other countries enter, live, and remain in the United States. Parts of the Constitution address related subjects. The Supreme Court has sometimes relied upon Congress's enumerated powers over naturalization and foreign commerce, and to a lesser extent upon the Executive's implied Article II foreign affairs power, as sources of federal immigration power. Significantly, however, the Court has also consistently attributed the immigration power to the federal government's inherent sovereign authority to control its borders and its relations with foreign nations. This inherent sovereign power, according to the Court, gives Congress essentially unfettered authority to restrict the entry of nonresident aliens. The Court has determined that the executive branch, by extension, possesses unusually broad authority to enforce laws pertaining to alien entry, and to do so under a level of judicial review much more limited than that which would apply outside of the exclusion context. The scope of the federal government's power to exclude aliens is at the forefront of litigation concerning two successive executive orders (the second revising and replacing the first) issued by President Donald Trump that, in their revised form, seek to deny entry temporarily to foreign nationals from six predominantly Muslim countries and to all refugees, subject to limited waiver. A series of lower court decisions largely enjoined implementation of the orders. In particular, courts ruled that the President's travel restrictions are likely unconstitutional or exceed his statutory authority. The Supreme Court granted certiorari to review decisions by the U.S. Courts of Appeals for the Fourth and Ninth Circuits that upheld nationwide injunctions against the revised executive order. The Supreme Court also partially stayed the injunctions, allowing the executive branch to implement the revised order in part pending the outcome of the litigation. A decision by the Court on the merits of these cases, which have come to be known as the "Travel Ban" litigation, could deliver a major statement about the constitutional and statutory scope of the President's power to exclude aliens. The litigation has threshold issues, however, such as questions of mootness and standing, that may well prevent the Court from reaching the merits, particularly following the issuance of a presidential proclamation superseding and somewhat modifying some of the entry restrictions at issue in the case. This report provides an overview of the legislative and executive powers to exclude aliens. First, the report discusses a gatekeeping legal principle that frames those powers: nonresident aliens outside the United States cannot challenge their exclusion from the country. Second, the report analyzes the extent to which the constitutional and statutory rights of U.S. citizens limit the exclusion power under the "facially legitimate and bona fide" test of Kleindienst v. Mandel . The report concludes with a case study. The report applies the principles of the Supreme Court's immigration jurisprudence to the two primary claims that U.S. persons and entities have pressed against the President's revised executive order in the "Travel Ban" litigation: (1) that the revised order violates the Establishment Clause; and (2) that the revised order violates the Immigration and Nationality Act (INA). As discussed later in the report, case law on the exclusion of aliens has come to focus upon whether the rights of U.S. citizens limit the government's power to exclude. The case law arrived at this issue, however, only after the Supreme Court had worked out an essential underlying principle: nonresident aliens outside the United States have no constitutional or statutory rights with respect to entry and therefore no legal grounds to challenge their exclusion. The Supreme Court developed this principle in a series of cases between the late 19 th and mid-20 th centuries about aliens denied admission after arriving by sea. The law at the time allowed such aliens to challenge the legality of their exclusion by filing a petition for habeas corpus in federal district court. This procedural right, however, proved hollow. In every case, the Supreme Court determined that the federal government's sovereign prerogative to forbid the entry of foreigners foreclosed the aliens' claims. In one famous case, the Court declared itself powerless to review the government's decision to exclude—without any explanation other than that the entry would be "prejudicial"—the German bride of a U.S. World War Two veteran. In another case, the Court refused to question the government's undisclosed reasons for denying entry to an essentially stateless alien returning to the United States after a prior period of residence, even though the exclusion relegated the stateless alien to indefinite detention on Ellis Island. The Supreme Court has since relied upon these cases for the proposition that excluded nonresident aliens cannot state legal claims with respect to entry. The related doctrine of consular nonreviewability precludes judicial review of visa denials. This doctrine developed in the lower courts and appears to derive from the line of Supreme Court cases described above. The consular nonreviewability doctrine, like the rule foreclosing alien claims itself, does not have a clearly enunciated theoretical foundation. It does have a clear consequence, however: the millions of nonresident aliens denied visas each year at U.S. consulates abroad cannot themselves challenge the visa denial in federal court. The Supreme Court has never endorsed the doctrine by name, but the Court has twice stated (without holding) that nonresident aliens cannot challenge visa denials. Put simply, under the consular nonreviewability doctrine and the related Supreme Court cases, nonresident aliens located outside the United States cannot challenge their exclusion from the country in federal court. The rule against alien challenges to denials of entry becomes more convoluted in the context of arriving aliens denied entry at the border, notwithstanding the rule's provenance in cases about such aliens. Arriving alien cases, unlike visa cases, can involve people stranded at the nation's threshold and, accordingly, sometimes present issues that go beyond the denial of entry itself to implicate the government's treatment of persons under its control. Whether the Constitution protects arriving aliens from unduly burdensome enforcement measures, such as prolonged detention pending removal, remains a disputed question. But Congress has provided certain statutory protections in this regard. Moreover, arriving aliens have succeeded in obtaining judicial review of statutory claims against detention or interdiction at sea by U.S. authorities. Such review for aliens pressing claims against the denial of visas abroad, in contrast, does not exist. To be sure, the exclusion power with respect to arriving aliens remains vast, and Congress relied on this broad power in 1996 when it created expedited removal procedures for many arriving aliens that limit or foreclose judicial review. But the exclusion power applies with its maximum force, in practice if not in theory, only to aliens who apply to enter from abroad, because such applications isolate the issue of denial of entry from the treatment issues that physical presence at the border or inside the country sometimes triggers. It is this core of the exclusion power that the remainder of this report addresses and that the Travel Ban cases, which concern the exclusion of aliens generally located far from the nation's shores, implicate most directly. Even as applied to aliens abroad, the rule against nonresident alien challenges to denials of entry has a major limitation: the rule only forecloses challenges brought by nonresident aliens themselves. Thus, if a U.S. citizen claims that the exclusion of an alien violated the U.S. citizen ' s rights , the rule against alien challenges does not apply. Cases that invoke this limitation account for the entirety of the Supreme Court's modern exclusion jurisprudence. The Court has not considered a nonresident alien's own challenge to a denial of entry in decades. The question about the extent to which U.S. citizens can challenge an alien's exclusion, on the other hand, has occupied the Court in three important cases since 1972: Kleindienst v. Mandel , Fiallo v. Bell , and the splintered Kerry v. Din . Under the rule that these cases establish, the government need only articulate a "facially legitimate and bona fide" reason for excluding a nonresident alien or class of aliens in order to prevail against an American citizen's claim that his or her constitutional rights have been violated by the exclusion. This section of the report discusses the three exclusion cases, an understanding of each of which is fundamental to this area of the Supreme Court's jurisprudence. Next, the section discusses the cases' implications for the scope of congressional power to exclude aliens and the scope of the concomitant executive power.