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 Missouri, as in all states, habeas corpus is a fundamental legal procedure that protects individuals from being detained indefinitely without just cause. This right is enshrined in the U.S. Constitution and applies to both federal and state detentions. If a person is detained by the federal government in Missouri, they can file a habeas corpus petition to challenge their detention. Upon filing, the government is required to present to a neutral judge a valid reason for the individual's imprisonment. The petition is treated as a civil action against the government official, typically the prison warden, who is holding the individual. The burden is on the government to justify the detention, and if they fail to provide sufficient cause, the judge may order the release of the detainee. This process serves as a critical check on governmental power and upholds the right to due process guaranteed by the Constitution.