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 Illinois, as in all states, the principle of habeas corpus is a critical component of the legal system, providing a means to challenge unlawful detention. Habeas corpus, enshrined in Article I, Section 9, Clause 2 of the U.S. Constitution, is a federal right that applies to all individuals within the United States, including those in Illinois. Under this provision, individuals detained by the federal government can file a habeas corpus petition in federal court. This petition obligates the government to present a valid legal basis for the individual's detention before a neutral judge. If the government cannot justify the detention, the judge can order the release of the detainee. The process is civil in nature and is typically directed against the government official responsible for the detention, such as the prison warden. It is important to note that while habeas corpus primarily applies to federal detentions, Illinois state law also provides similar protections against unlawful state detentions under the Illinois Habeas Corpus Act (735 ILCS 5/10-101 to 5/10-150), allowing individuals to challenge their state-level detention in Illinois courts.