law that retroactively makes criminal conduct that was not criminal when performed, increases the punishment for crimes already committed, or changes the rules of procedure in force at the time an alleged crime was committed in a way substantially disadvantageous to the accused.

The Constitution of the United States (Constitution of the United States of America) forbids Congress and the states to pass any ex post facto law. In 1798 it was determined that this prohibition applies only to criminal laws and is not a general restriction on retroactive legislation. Implicit in the prohibition is the notion that individuals can be punished only in accordance with standards of conduct that they might have ascertained before acting. The clause also serves, in conjunction with the prohibition of bills of attainder, as a safeguard against the historic practice of passing laws to punish particular individuals because of their political beliefs. In 1867, in Cummings v. Missouri and Ex parte Garland, the United States Supreme Court (Supreme Court of the United States) condemned as both bills of attainder and ex post facto laws the passage of post- American Civil War loyalty-test oaths, which were designed to keep Confederate sympathizers from practicing certain professions.

The policies underlying ex post facto laws are recognized in most developed legal systems, reflected in the civil law maxim nulla poena sine lege (“no punishment without law”), a principle whose roots are embedded in Roman law. In England Parliament is not prohibited from passing ex post facto laws. However, following the common-law (common law) tradition, judges have refused to interpret legislation retroactively unless Parliament has clearly expressed such an intention.

The prosecution of Nazi (Nazi Party) leaders at the Nürnberg trials following World War II for the crime of aggressive war—a crime specifically defined for the first time in the Allied charter creating the International Military Tribunal for war criminals—provoked extensive discussion over the scope and applicability of the principle against retroactive criminal laws.