Legal Definitions - retroactive law

A retroactive law is a type of legislative act that affects acts or facts that existed before the law came into effect. It looks backward or contemplates the past. However, a retroactive law is not unconstitutional unless it:

1. Is in the nature of an ex post facto law or a bill of attainder
2. Impairs the obligation of contracts
3. Divests vested rights
4. Is constitutionally forbidden

For example, let's say a new law is passed that makes it illegal to own a certain type of car. If the law is retroactive, it means that people who already own that type of car could be punished for something that was legal when they bought it. This would be unconstitutional because it impairs the obligation of contracts and divests vested rights.

Another example is if a law is passed that makes it illegal to do something that was legal in the past. This would be unconstitutional if it is in the nature of an ex post facto law or a bill of attainder, which means punishing someone for something that was legal when they did it.