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 Montana, 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 individuals cannot be held indefinitely without just cause. Under Article I, Section 9, Clause 2 of the U.S. Constitution, the right to petition for a writ of habeas corpus is a protection against government overreach that cannot be suspended except in cases of rebellion or invasion where public safety may require it. When a person detained by the federal government in Montana files a habeas corpus petition, the government is compelled to present a valid reason for the individual's detention to a neutral judge. This process is treated as a civil matter, and the petition is typically filed against the government agent responsible for the detainee's custody, often the prison warden. The Montana state statutes also provide mechanisms for filing habeas corpus petitions, which must comply with both federal law and state procedural rules.