Habeas corpus ( (listen); from Medieval Latin, lit. 'that you have the body') is a recourse in law through which a person can report an unlawful detention or imprisonment to a court and request that the court order the custodian of the person, usually a prison official, to bring the prisoner to court, to determine whether the detention is lawful.

The writ of habeas corpus was described in the eighteenth century by William Blackstone as a "great and efficacious writ in all manner of illegal confinement". It is a summons with the force of a court order; it is addressed to the custodian (a prison official, for example) and demands that a prisoner be brought before the court, and that the custodian present proof of authority, allowing the court to determine whether the custodian has lawful authority to detain the prisoner. If the custodian is acting beyond their authority, then the prisoner must be released. Any prisoner, or another person acting on their behalf, may petition the court, or a judge, for a writ of habeas corpus. One reason for the writ to be sought by a person other than the prisoner is that the detainee might be held incommunicado. Most civil law jurisdictions provide a similar remedy for those unlawfully detained, but this is not always called habeas corpus. For example, in some Spanish-speaking nations, the equivalent remedy for unlawful imprisonment is the amparo de libertad ("protection of freedom").

Habeas corpus has certain limitations. Though a writ of right, it is not a writ of course. It is technically only a procedural remedy; it is a guarantee against any detention that is forbidden by law, but it does not necessarily protect other rights, such as the entitlement to a fair trial. So if an imposition such as internment without trial is permitted by the law, then habeas corpus may not be a useful remedy. In some countries, the writ has been temporarily or permanently suspended under the pretext of a war or state of emergency, for example by Abraham Lincoln during the American Civil War (see Habeas Corpus Suspension Act (1863)) and in England in 1795

The right to petition for a writ of habeas corpus has nonetheless long been celebrated as the most efficient safeguard of the liberty of the subject. The jurist Albert Venn Dicey wrote that the British Habeas Corpus Acts "declare no principle and define no rights, but they are for practical purposes worth a hundred constitutional articles guaranteeing individual liberty".

The writ of habeas corpus is one of what are called the "extraordinary", "common law", or "prerogative writs", which were historically issued by the English courts in the name of the monarch to control inferior courts and public authorities within the kingdom. The most common of the other such prerogative writs are quo warranto, prohibito, mandamus, procedendo, and certiorari. The due process for such petitions is not simply civil or criminal, because they incorporate the presumption of non-authority. The official who is the respondent must prove their authority to do or not do something. Failing this, the court must decide for the petitioner, who may be any person, not just an interested party. This differs from a motion in a civil process in which the movant must have standing, and bears the burden of proof.