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[case]Calder v. Bull[Calder v. Bull] (1798), where plaintiffs challenged the right of a state legislature to legislate against their interests in a civil case. Justice Samuel Chase ruled, however, that the ex post facto exclusion applied only to criminal not to civil law.

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[case]Test Oath Cases[Test Oath Cases] (1867), the court voided laws requiring oaths regarding past loyalties that disqualify individuals from holding office or practicing certain professions. In Beazell v. Ohio[case]Beazell v. Ohio[Beazell v. Ohio] (1925), changes in laws regarding evidence that “operate only in a limited and unsubstantial manner to [the defendant’s] disadvantage, are not prohibited.” In Weaver v. Graham[case]Weaver v. Graham[Weaver v. Graham] (1981), the Court held that laws that reduce a prisoner’s “good time” credits, thereby extending punishment, are prohibited, but those acting to defendants’ benefit are allowed. The Court has also held that deportation of aliens and the denial of a passport are not banned as ex post facto because they are not criminal punishments.

Bill of attainder

Calder v. Bull

Chase, Samuel