An ex post facto law is a law that retroactively changes the legal consequences of actions that were committed, or relationships that existed, before the enactment of the law. In a legal context, ex post facto is most typically used to refer to a criminal statute that punishes actions retroactively, thereby criminalizing conduct that was legal when originally performed. The United States Constitution prohibits ex post facto laws in two clauses: Article 1, § 9, which prohibits Congress from passing any laws which apply ex post facto, and Article 1 § 10, which prohibits the states from passing any laws which apply ex post facto.

Ex post facto laws can be categorized into three types: those that punish an act previously committed, which was innocent when done; those that make more burdensome the punishment for a crime, after its commission; and those that deprive one charged with crime of any defense available according to law at the time when the act was committed. The prohibition against ex post facto laws applies only to legislative action and does not touch erroneous or inconsistent decisions by the courts.

In some nations that follow the Westminster system of government, such as the United Kingdom, ex post facto laws are possible, because the doctrine of parliamentary supremacy allows Parliament to pass any law it wishes. However, in a nation with an entrenched bill of rights or a written constitution, ex post facto legislation may be prohibited or allowed, and this provision may be general or specific.

In summary, an ex post facto law is a law that retroactively changes the legal consequences of actions that were committed before the enactment of the law. The United States Constitution prohibits ex post facto laws, and they can be categorized into three types. While ex post facto laws are possible in some nations, they may be prohibited or allowed in others, depending on the countrys legal system.