Ex Post Facto Clause
[U.S. Const., Article I, § 9, cl. 3; Article I, § 10, cl. 1] Prohibits Congress and the states from enacting criminal laws that apply retroactively in one or more of the following ways: makes an action criminal that was not criminal when committed; increases the seriousness of a crime or the severity of the punishment; changes the rules of evidence in a manner that makes it easier to convict a defendant.