What is habeas corpus?

Habeas corpus, Latin for that you have the body, is a judicial order directing whoever holds a prisoner to produce them before a court and justify the detention. Its function is narrow and fundamental: it does not ask whether the detainee is guilty but whether the confinement is lawful. It descends from English common law, was reinforced by the Habeas Corpus Act of 1679, and was so established by the founding that the Constitution does not create it but instead limits its suspension, providing that the privilege shall not be suspended unless in cases of rebellion or invasion the public safety may require it.

In modern American practice it operates principally as a route for state prisoners to raise federal constitutional claims after direct appeals are exhausted, arguing for example ineffective assistance of counsel, prosecutorial misconduct, or newly discovered evidence of innocence. Congress substantially restricted that path in the Antiterrorism and Effective Death Penalty Act of 1996, imposing a one-year filing deadline, sharply limiting second petitions, and requiring federal courts to defer to state court determinations unless they were contrary to or an unreasonable application of clearly established federal law, which is a demanding standard.

Suspension has occurred rarely. Lincoln suspended it during the Civil War, prompting a constitutional dispute over whether that power belongs to Congress or the president, and it has been suspended in limited circumstances a handful of other times. In the post-2001 period the Supreme Court held in Boumediene v. Bush in 2008 that detainees at Guantanamo Bay retain a constitutional right to habeas review, striking down a statutory attempt to remove it. The writ is often described as the great writ precisely because it is the mechanism that makes other rights enforceable: a right not to be detained arbitrarily means little without a court empowered to demand a reason.