The Immigration and Nationality Act (INA) authorizes-and in some cases requires-the Department of Homeland Security (DHS) to detain non-U.S. nationals (aliens) arrested for immigration violations that render them removable from the United States. An alien may be subject to detention pending an administrative determination as to whether the alien should be removed, and, if subject to a final order of removal, pending efforts to secure the alien's removal from the United States. The immigration detention scheme is multifaceted, with different rules that turn on several factors, such as whether the alien is seeking admission into the United States or has been lawfully admitted into the country; whether the alien has engaged in certain proscribed conduct; and whether the alien has been issued a final order of removal. In many instances DHS maintains discretion to release an alien from custody. But in some instances, such as when an alien has committed specified crimes, the governing statutes have been understood to allow release from detention only in limited circumstances. Various provisions confer substantial authority upon DHS to detain removable aliens, but that authority has been subject to legal challenge, particularly in cases involving the prolonged detention of aliens without bond. DHS's detention authority is not unfettered, and due process considerations may inform the duration and conditions of aliens' detention. In 2001, the Supreme Court in Zadvydas v. Davis construed the statute governing the detention of aliens following an order of removal as having implicit, temporal limitations. The Court reasoned that construing the statute to permit the indefinite detention of lawfully admitted aliens after their removal proceedings would raise "serious constitutional concerns." In 2003, however, the Court in Demore v. Kim ruled that the mandatory detention of certain aliens pending their removal proceedings, at least for relatively brief periods, was constitutionally permissible. The interplay between the Zadvydas and Demore rulings has called into question whether the constitutional standards for detention prior to a final order of removal differ from those governing detention after a final order is issued. Several lower courts have interpreted Demore to mean that mandatory detention pending removal proceedings is not per se unconstitutional, but that Zadvydas cautions that if this detention becomes "prolonged" it may not comport with due process requirements. Additionally, some lower courts have recognized constraints on DHS's detention power that the Supreme Court has not yet considered. For instance, some courts have ruled that the Due Process Clause requires aliens in removal proceedings to have bond hearings when detention becomes prolonged, where the government bears the burden of proving that the alien's continued detention is justified. In addition, a settlement agreement known as the "Flores Settlement," which is enforced by a federal district court, currently limits DHS's ability to detain alien minors who are subject to removal. Further, while litigation concerning immigration detention has largely centered on the duration of detention, some courts have considered challenges to the conditions of immigration confinement, generally under the standards applicable to pretrial detention in criminal cases. Some courts have also restricted DHS's ability to take custody of aliens detained by state or local law enforcement officials upon issuance of "immigration detainers." In short, while DHS generally has broad authority over the detention of aliens, that authority is not without limitation. As courts continue to grapple with legal and constitutional challenges to immigration detention, Congress may consider legislative options that clarify the scope of the federal government's detention authority.