English legal commentator William Blackstone described the writ of habeas corpus as a second Magna Carta, and Supreme Court Chief Justice John Marshall called it the "great writ." It has been part of the Anglo-American common law tradition since the Middle Ages. In the United States, it has been a source of tension between state and federal courts, and a point of controversy with respect to the separate powers of the legislative, executive, and judicial branches. It is very much in the news today as the Supreme Court considers whether the writ of habeas corpus is available to the detainees at Guantanamo Bay, Cuba. The basic purpose of the writ of habeas corpus is to afford a person who has been detained the chance to challenge the legality of his or her detention. The writ has a rich and varied history, and the scope of the writ has changed over the centuries of its use. This article looks at the origins of the writ, its development in English and American law, and current points of controversy regarding the writ. (Contains 6 notes.)