Statutes that render an act committed in the past illegal, although that act was legal when it took place.
Ex post facto laws, regarded as tools of oppression when the U.S. Constitution was written, were banned in Article I, section 9, of the Constitution, where they are excluded from the powers of Congress, and in Article I, section 10, where the exclusion is applied to state legislatures.
The Supreme Court specified the extent and meaning of this restriction on the powers of U.S. government. Its principal interpretation came in Calder v. Bull
Besides laws criminalizing actions that were committed before the law was passed, Chase’s decision excluded laws that make crimes greater or subject to more severe punishments than when committed and laws that alter rules of evidence, making conviction easier.
Later Court cases extended the ex post facto restriction. In the Test Oath Cases
Bill of attainder
Calder v. Bull