Ex post facto law

An ex post facto law or retrospective law is a law that is retroactive, i.e. it affects facts or legal relationships that have existed prior to the enactment of the law. Ex post facto is a term from the Latin and means "after the deed." In reference to crminal law, it is an attempt to criminalize an action that was not a crime when it was done. A law may have an ex post facto effect without being technically ex post facto. For example, when a law that repeals another law, the repealed legislation will no longer apply to situations even when those situations arose before the law was repealed.

Generally speaking, ex post facto laws are seen as a violation of the rule of law as it applies in a free and democratic society. Most common law jurisdictions do not permit retrospective legislation, though some have suggested that judge-made law is retrospective as a new precedent applies to events that happened prior to the judicial decision. In countries that have a parliamentary system of government, ex post facto laws are technically possible as parliamentary supremacy allows the parliament to pass any law it wishes; an entrenched bill of rights may limit such supremacy or a written constitution may prevent such ex post facto legislation.

In the United States ex post facto laws are explicitly prohibited by Article I section 9 (applying to federal law) and section 10 (applying to state law) of the U.S. Constitution.