Ex post facto law

An ex post facto law (corrupted Latin, literally "out of the aftermath") is a law that retroactively changes the legal consequences of actions that were committed before the enactment of the law.

This is considered problematic for various reasons, such as when criminalizing actions stated to be legal when they were done.

While American jurisdictions generally prohibit ex post facto laws, European countries apply the principle of lex mitior ("the milder law"). It provides that, if the law has changed after an offense was committed, the version of the law that applies is the one that is more advantageous for the accused. This means that ex post facto laws apply in European jurisdictions to the extent that they are the milder law.

Ex post facto laws were notably used at the notorious Nuremberg trials.

Demands for various reparations often involve demands for ex post facto laws.