Also known as: ex post facto law · ex-post-facto legislation · ex post facto laws · retroactive legislation · ex post facto
Written by attorneys · grounded in primary & secondary sources — see below
A constitutional prohibition barring legislative bodies from enacting criminal laws that retroactively criminalize conduct lawful when performed or that increase punishment for past acts. The prohibition safeguards fair notice by ensuring individuals can rely on existing criminal rules when ordering their affairs. It applies to both federal and state governments through express textual commands.
Select any source to read its text and confirm it supports the definition.
Eileen Epstein operated a manufacturing plant in State A for five years under permits that allowed unlimited carbon output. After new federal standards took effect, State A passed a statute making any prior exceedance of the new ceilings a criminal offense punishable by fines and imprisonment. Eileen faces prosecution solely for her pre-enactment conduct. The statute cannot apply to her because it retroactively converts lawful emissions into a crime.
Emma Erickson was investigated for securities fraud but the five-year limitations period expired without charges. Congress later extended the period to eight years and applied it to already-barred conduct. Prosecutors now seek to indict Emma under the new deadline. The extension violates the prohibition because it reopens the possibility of punishment for conduct that had become immune.
Elise Everly, a telecom executive, directed her company to use a foreign vendor during a period when federal law expressly permitted the arrangement. Congress later enacted a criminal statute making that prior use an offense carrying mandatory prison time. Elise is indicted solely on the basis of her pre-enactment decisions. The statute cannot stand because it retroactively criminalizes conduct that was lawful when undertaken.
Esther Eisenberg was convicted under a narrow interpretation of a fraud statute that existed at the time of her conduct. A later Supreme Court decision broadened the statute's reach and prosecutors seek to apply the new reading to Esther's already-completed acts. The expanded interpretation cannot be used against her because it functions as an unexpected legislative-style change that increases criminal exposure after the fact.
Erika Echevarria was charged with trespass after sitting at a lunch counter for an extended period. At the time of her conduct the state trespass statute had been consistently interpreted to require refusal of a direct order to leave. A subsequent state supreme court ruling eliminated that requirement and prosecutors apply the new reading to Erika. The reinterpretation cannot support her conviction because it retroactively alters the elements of the offense in an unforeseeable way.
Emily Ellis received a sentence under the criminal code in force when she committed her offense. Congress later amended the code to impose a harsher mandatory minimum for the same conduct and the government seeks to resentence Emily under the new provision. The amendment cannot increase her punishment because it would attach greater consequences to acts already completed.
It forbids any criminal statute that makes previously lawful conduct a crime, increases the punishment for past conduct, or alters rules of evidence to make conviction easier. The core concern is that individuals must receive fair notice of criminal liability before they act.
Supporting sources
…ample funds to secure one, the expedient of giving a warning is too simple and the rights involved too important to engage in ex post facto inquiries into financial ability when there is any doubt at all on that score. [^maj-44]: If an individual indicates his desire to remain silent, but has an attorney present, there may be…
…The Court rested the invalidation on alternative sources of authority: the specific prohibitions against bills of attainder, ex post facto laws, laws
…321. [^maj-12]: The significance of this pronouncement cannot be minimized. The holding that retrial by a jury of eight was an ex post facto law is perforce built upon the conclusion that the jury of 12 was a right of substance. If the right were merely a procedure mandated by statute, it would not have required the ex
…here, he argued that the honest-services statute, § 1346, is unconstitutionally vague and that his conviction violated the Ex Post Facto
…(1977) (holding that changes in the death penalty statute between the time of the murder and the sentencing did not amount to an ex post facto
Yes. The clause targets penal legislation. Civil regulatory measures that attach future consequences to past facts generally survive unless they are so punitive in purpose or effect that they function as criminal punishment.
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Directly no, because the clause constrains legislatures. Due process, however, bars courts from applying an unforeseeable judicial expansion of criminal liability to conduct that occurred before the decision.
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Yes when applied to already time-barred criminal prosecutions. The extension retroactively removes a complete defense and reopens the possibility of conviction for past conduct.
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The prohibition addresses retroactive criminal liability and punishment. The Contracts Clause separately prevents states from substantially impairing existing contractual obligations without sufficient justification.
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