Nulla poena sine lege

Nulla poena sine lege (Latin: "no penalty without a law"): in Law, the principle that one cannot be penalised for doing something that isn't prohibited by law. This principle is accepted as just and upheld by the penal codes of virtually all modern democracies. It is related to the principle called "Nullum crimen, nulla poena sine praevia lege poenali", which means penal law cannot be enacted retroactively.

The question of jurisdiction may sometimes come to contradict this principle. For example, customary international law allows the persecution of pirates by any country (applying universal jurisdiction), even if they did not commit crimes at the area that falls under this country's law. A similar principle has appeared in the recent decades, with regard to crimes of genocide, allowing the Nuremberg trials to take place. However, it seems that universal jurisdiction is not to be expanded substancially to other crimes, so that to satisfy Nulla poena sine lege.