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The phrase Nulla poena sine lege (Latin: "no penalty without a law") refers to the legal 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.
One complexity is the lawmaking power of judges under common law. Even in civil law systems that do not admit judge-made law, when does the function of interpretation of the criminal law end and judicial lawmaking begin?
The question of jurisdiction may sometimes come to contradict this principle. For example, customary international law allows the prosecution 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 substantially to other crimes, so that to satisfy Nulla poena sine lege.
Theoretically speaking, defenders of universal jurisdiction argue that this exercise does not violate nulla poena sine lege, since these acts, even if not prohibited under the law of any country, are in violation of international law, which many legal theorists view as being equally law.
Natural law theorists or divine command theorists would further add that nulla poena sine lege is not violated if the punished act is against natural law or the law of God, respectively, even if it violates no positive law.
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