Define: Ex Post Facto Law

An ex post facto law is a law that retroactively applies to actions that were legal when they were committed. This type of law is prohibited in criminal cases under the U.S. Constitution, but it may be permitted in civil cases. A retroactive law is a legislative act that applies to past events and impacts acts or facts that occurred before the law was enacted. It is only considered unconstitutional if it violates contract obligations, takes away vested rights, or is explicitly prohibited by the constitution.

An ex post facto law refers to a law that is applied retroactively to an action that was legal at the time it occurred, particularly if it negatively impacts a person’s rights. For instance, if a law is enacted today that declares driving a red car illegal, and someone who drove a red car yesterday is punished for it, that would be considered an ex post facto law. The U.S. Constitution prohibits ex post facto criminal laws, although retrospective civil laws may be permissible. A retrospective law is a legislative measure that looks back or considers the past, affecting acts or facts that existed prior to the law’s implementation. A retroactive law is not unconstitutional unless it (1) resembles an ex post facto law or a bill of attainder, (2) impairs contractual obligations, (3) deprives vested rights, or (4) is constitutionally prohibited. For example, if a law is passed that alters the tax rate for the previous year, it would be considered a retrospective law, but not an ex post facto law since it does not criminalize an action that was legal at the time it occurred.

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This glossary post was last updated: 17th April 2024.