How does habeas corpus allow a prisoner to challenge detention?

A writ of habeas corpus, often called the "Great Writ," is a fundamental common law tool used to protect individual liberty against unlawful state action. It functions by requiring a custodian—such as a prison official or police officer—to bring a detainee before a judge to assess the legality of their detention.12

Rather than determining guilt or innocence, the writ focuses on whether the custody is lawful under constitutional or statutory provisions. Prisoners may use this process to challenge their detention on several grounds, including:12

• Pre-trial issues: Challenging excessive bail or requiring police to formally charge an individual or release them.2
• Conviction challenges: Arguing that a conviction resulted from illegally obtained evidence, ineffective assistance of counsel, or an improperly selected jury.1
• Post-sentence issues: Seeking release if held unlawfully after a sentence has expired.2
• Extradition: Contesting the validity of an extradition warrant.2

This mechanism ensures that the government remains accountable to the rule of law, preventing arbitrary imprisonment.2