Also known as: ex post facto · ex-post-facto rule · ex post facto laws · retroactive criminal law prohibition
Written by attorneys · grounded in primary & secondary sources — see below
A constitutional prohibition barring legislatures from enacting criminal laws that retroactively criminalize conduct, increase punishment for past acts, or alter rules of evidence to the defendant's disadvantage. The rule ensures fair notice by preventing new or harsher penalties from attaching to completed conduct.
Ella Emerson completed a series of sales transactions in State A before any statute addressed them. State A later passed a law criminalizing those exact transactions and imposing prison terms. Ella faces prosecution under the new statute for her prior conduct.
Emanuel Escobar's alleged offense occurred years earlier and the statute of limitations had already expired. The legislature then extended the limitations period and applied it retroactively to his case. Prosecutors reopen the matter and seek to convict him under the revived window.
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Elena Estrada engaged in conduct that was lawful when performed. Congress later enacts a statute making that same conduct a federal crime and directs that it apply to all prior instances. Federal authorities indict her solely on the basis of her pre-enactment actions.
Ethan Evans published a work under an existing copyright term. Congress later lengthens the term and applies the extension to already-published works. Ethan claims the change retroactively alters his legal position in a manner forbidden by the ex post facto rule.
Eric Espinoza distributed materials under the prevailing obscenity test at the time. A later Supreme Court decision announces a new standard and prosecutors attempt to apply it to his earlier distribution. Eric objects that the new test cannot govern his completed conduct.
Ewan Eckhart held a legal status under the law in force when he acted. A subsequent decision recharacterizes that status and imposes new disabilities on the prior conduct. Ewan contends the change violates the ex post facto rule by punishing him for acts lawful at the time.
The rule bars legislatures from making conduct criminal after the fact, increasing punishment for completed acts, or changing evidentiary rules to make conviction easier. These changes all operate retroactively on past conduct.
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The prohibition applies to criminal or penal legislation. Civil regulatory measures generally fall outside its scope unless they are so punitive in purpose or effect that they function as punishment.
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Yes. Extending a limitations period after it has already expired and applying the extension to revive a time-barred prosecution retroactively alters the legal consequences of past conduct and violates the clause.
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No. The rule targets only retroactive criminal punishment. Prospective changes and most civil measures remain permissible even when they attach new consequences to past facts.
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…a measure was "necessary and proper" to the discharge of its substantive legislative authority, pass a bill of attainder or ex post facto law contrary to the prohibitions contained in § 9 of Art. I. No more may it vest in itself, or in its officers, the authority to appoint officers of the United States when the Appointments…