This article traces the transformation of martial law during the Civil Wars and Interregnum culminating with the creation of the High Courts of Justice in the 1650s. The Long, Rump, and Protectorate parliaments used, adapted, and combined martial law procedures with others to solve some of the most difficult and pressing legal problems they faced. These problems included the trial of spies, traitors to the parliamentary cause, Charles I and his royalist commanders of the Second Civil War, and conspirators, plotters, and rebels during the 1650s. The Long Parliament, the English Commonwealth, and the Protectorate governments used these legal innovations to control discretion at law, and to terrorize dissidents into obedience. The Petition of Right, whose makers had demanded that English subjects only be tried by life and limb by their peers in peacetime, was overturned in order to meet these challenges.