An ex post facto law is a law that retroactively changes the legal consequences (or status) of actions that were committed, or relationships that existed, before the enactment of the law. In criminal law, it may criminalize actions that were legal when committed; it may aggravate a crime by bringing it into a more severe category than it was in when it was committed; it may change the punishment prescribed for a crime, as by adding new penalties or extending sentences; or it may alter the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed. Conversely, a form of ex post facto law commonly called an amnesty law may decriminalize certain acts or alleviate possible punishments (for example by replacing the death sentence with lifelong imprisonment) retroactively. Such laws are also known by the Latin term in mitius.
A law may have an ex post facto effect without being technically ex post facto. For example, when a law repeals a previous law, the repealed legislation is no longer applicable to situations to which it previously was, even if such situations arose before the law was repealed. The principle of prohibiting the continued application of such laws is called Nullum crimen, nulla poena sine praevia lege poenali, especially in European Continental systems.
Some common-law jurisdictions do not permit retroactive criminal legislation, though new precedent generally applies to events that occurred before the judicial decision. Ex post facto laws are expressly forbidden by the United States Constitution in Article 1, Section 9, Clause 3. In some nations that follow the Westminster system of government, such as the United Kingdom, ex post facto laws are technically possible, because the doctrine of parliamentary supremacy allows Parliament to pass any law it wishes. In a nation with an entrenched bill of rights or a written constitution, ex post facto legislation may be prohibited.