Ellingburg v. United States · ¶24

The Constitution twice prohibits ex post facto laws. As to the Federal Government, it provides that “No Bill of Attainder or ex post facto Law shall be passed.” Art. I, § 9, cl. 3. As to the States, it provides that “No State shall . . . pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts.” Art. I, § 10, cl. 1. In its general sense, a law is ex post facto—meaning “after the fact”—when it applies retroactively to conduct that occurred before the law was enacted.

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