Ex post facto law

An ex post facto law retroactively criminalizes conduct, increases punishment, or otherwise imposes a prohibited retrospective penalty.

Ex post facto clause: A constitutional provision barring specified forms of retroactive penal legislation. This is the constitutional rule that gives the prohibition legal force.
Calder v. Bull (1798): The Supreme Court case in which Justice Samuel Chase listed four kinds of prohibited ex post facto laws. The decision is the principal starting point for applying the U.S. constitutional ban.
Ex post facto clause of the United States Constitution: Article I provisions barring Congress and the states from enacting ex post facto laws. The U.S. constitutional text provides the specific prohibition discussed on this page.
English Bill of Rights 1689: An English statute that restricted royal power and declared certain abuses of government unlawful. Its rejection of retrospective penal practices helped shape the American prohibition.
Calder v. Bull: A 1798 U.S. Supreme Court case defining categories of prohibited ex post facto laws. Its opinion supplied the classic four-part test for identifying prohibited laws.
Smith v. Doe: A 2003 U.S. Supreme Court case holding that Alaska's sex-offender registration scheme was not punitive for ex post facto purposes. It shows how courts distinguish civil regulation from retroactive punishment.
Bill of attainder: A law that imposes punishment on named people or an identifiable group without a judicial trial. It prohibits legislative punishment directly, rather than punishment made retroactive.
Constitutional Convention: The 1787 meeting in Philadelphia that drafted the United States Constitution. The convention placed ex post facto prohibitions on both federal and state governments.
Retroactive law: A law that changes the legal consequences of events occurring before its enactment. Ex post facto laws are a narrower, penal subset of retroactive legislation.
Kennedy v. Mendoza-Martinez: A 1963 U.S. Supreme Court case setting factors for deciding whether a civil measure is punitive. Its factors help determine when a measure labeled civil triggers the prohibition.

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