Habeas corpus—also known as the Great Writ—is the legal procedure that prevents the federal government from arresting and holding (in prison) a person indefinitely without showing cause for detaining the person. This Latin term is pronounced "Hay-bee-us Corp-us."

A person detained by the federal government may challenge their detention by filing a habeas corpus petition—which then requires the government to explain to a neutral judge the justification for detaining the person. A habeas petition is prosecuted or litigated as a civil (noncriminal) matter against the government agent (usually the prison warden) who holds the defendant in custody.

The Founders of the United States who wrote the U.S. Constitution believed so strongly in this protection against government overreach they included it in the first article of the Constitution (Article I, Section 9, Clause 2).

In Massachusetts, as in all states, habeas corpus is a fundamental legal procedure protected by both the U.S. Constitution and state law. It serves as a safeguard against unlawful detention by the federal government, ensuring that no person can be held indefinitely without just cause. When a person is detained by the federal government, they have the right to file a habeas corpus petition, which obligates the government to present a valid reason for the detention to a neutral judge. This process is treated as a civil matter, with the petition typically filed against the government official responsible for the detainee's custody, often the prison warden. The right to habeas corpus is enshrined in Article I, Section 9, Clause 2 of the U.S. Constitution, reflecting the Founders' commitment to protecting individuals from government overreach. In Massachusetts, individuals may file a habeas corpus petition in federal court if they are in federal custody or in state court if they are in state custody.