In common law jurisdictions, habeas corpus, or more precisely habeas corpus ad subjiciendum, is a prerogative writ which requires the addressee to produce in court a person in its custody and justify his or her imprisonment. Known as the "Great Writ", the writ of habeas corpus could be used not only in criminal cases, but in cases of imprisonment for private debt. In many jurisdictions today the writ can also be issued against private individuals.

The words habeas corpus ad subjiciendum are Latin for "You (shall) have/hold the body to be subjected to (examination)", and are taken from the opening words of the writ in medieval times. Other habeas corpus writs also existed, e.g. habeas corpus ad testificandum, for the production of a prisoner to give evidence in court.

The right of habeas corpus has long been celebrated as the most efficient safeguard of the liberty of the subject (viz. by Dicey, who wrote that the Habeas Corpus Acts "declare no principle and define no rights, but they are for practical purposes worth a hundred constitutional articles guaranteeing individual liberty"), and the institution has been incorporated into the laws of several countries that do not follow the common law system. However, in most countries the right of habeas corpus can be suspended in time of national emergency.

Habeas Corpus is also the name of a play, a satirical, farcical black comedy by Alan Bennett. It details the story of a hedonistic family and the results of their dalliances.

Habeas corpus in the United States

This procedure, part of English common law, was considered important enough to be specifically mentioned in the U.S. Constitution, which says, "The Privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it." (Article One, section nine).

The most common American use of habeas corpus today is as part of the appeals process after conviction. Decisions by the Rehnquist Supreme Court and the Anti-Terrorism and Effective Death Penalty Act of 1996 have limited its use, especially in capital cases.

Suspension of habeas corpus during the American Civil War

Habeas corpus was suspended on April 27, 1861 during the American Civil War by President Lincoln in parts of midwestern states, including southern Indiana. He did so in response to demands by generals to set up military courts to rein in "Copperheads" or Peace Democrats, and those in the Union who supported the Confederate cause. His action was challenged in court and overturned by the US Circuit Court in Maryland (led by Supreme Court Chief Justice Roger B. Taney) in Ex Parte Merryman, 17 F. Cas. 144 (C.C.D. Md. 1861). Lincoln ignored Taney's order.

In 1864, Lambdin Milligan and four others were accused of planning to steal Union weapons and invade Union prisoner-of-war camps and were sentenced to hang by a military court. However, their execution was not set until May 1865, so they were able to argue the case after the Civil War. It was decided in the Supreme Court case Ex Parte Milligan 71 US 2 1866 that the suspension was unconstitutional because civilian courts were still operating, and the Constitution (according to the Court) provided for suspension of habeas corpus only if these courts are actually forced closed. This was one of the key Supreme Court Cases of the American Civil War, which dealt primarily with wartime civil liberties and martial law.

Civil War-era habeas corpus decisions have attracted renewed scrutiny since the beginning of the War on Terror, in light of the Bush Administration's assertion of presidential authority to designate even U.S. citizens as enemy unlawful combatants and hold them indefinitely, without criminal charges or access to counsel.