Ex Post Facto Clause Law and Legal Definition
Article I, section 10, clause 1 of the U.S. Constitution provides that no state shall pass any ex post facto law. Like wise, Article I, section 9, clause 3 imposes the same prohibition upon the federal government. The Constitution protects individuals by denying to the Congress or state legislatures the power to punish people by passing ex post facto laws. However, the Supreme Court has determined that these clauses prohibit laws with retroactive effect only in the field of criminal law and do not apply to to civil matters. Nonetheless, retroactive laws in the civil area may under certain circumstances violate the Contract or Due Process Clauses of the Constitution. The ban on ex post facto laws is a restraint on legislative power only and has no application to changes in the law made by judicial decision.