A statute that treats with facts or occurrences something that took place before the statute was enacted. While unusual, a retroactive law is only unconstitutional if it impairs vested rights, interferes with obligations under contracts (such as creating new obligations or attaching new disabilities), has the effect of an ex post facto law or bill of attainder, or is prohibited by the United States Constitution. Certain decisions granting new rights to criminal defendants under constitutional law have been given full retroactive effect. While others have been held to be effective only from the time of enactment forward. See also ex post facto and bill of attainder.