What is an ex post facto law?
An ex post facto law applies a new criminal rule to conduct that already happened, or raises the penalty after the fact. The Latin phrase means “after the fact.”
Congress.gov is the source of record for this guide. Its history of the Ex Post Facto Clause explains the ban on retroactive criminal punishment.
What would an ex post facto law do?
The key question is whether a new law makes the criminal consequences of past conduct worse. Two examples show the problem:
- Create a crime after the act. A legislature bans an activity today and tries to punish someone who did it legally last year.
- Increase a past offense’s penalty. A legislature raises a crime’s maximum sentence from one year to five years and applies that increase to an earlier offense.
A new criminal law that applies only to future conduct does not raise this retroactivity problem.
Does the ban cover Congress and the states?
Article I, Section 9 bars Congress from passing an ex post facto law. Article I, Section 10 bars the states from passing one. The state Ex Post Facto Clause overview explains how the same protection limits state legislatures.
Congress has lawmaking powers, but it must respect these limits. The enumerated powers guide explains the powers the Constitution expressly grants to Congress.
Is every retroactive law an ex post facto law?
No. The constitutional ban concerns criminal punishment, not every law that affects past events. Other constitutional limits can still apply to retroactive civil laws. Calling a law “civil” does not settle the question if the law actually imposes criminal punishment.
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