Principle of Legality

The principle of legality is the legal ideal that requires all law to be clear, ascertainable and non-retrospective. It requires decision makers to resolve disputes by applying legal rules that have been declared beforehand, and not to alter the legal situation retrospectively by discretionary departures from established law. It is closely related to legal formalism and the rule of law and can be traced from the writings of Feuerbach, Dicey and Montesquieu.

The principle has particular relevance in criminal and administrative law. In criminal law it can be seen in the general prohibition on the imposition of criminal sanctions for acts or omissions that were not criminal at the time of their commission or omission. The principle is also thought to be violated when the sanctions for a particular crime are increased with retrospective effect.

In administrative law it can be seen in the desire for state officials to be bound by and apply the law rather than acting upon whim. As such advocates of the principle are normally against discretionary powers.

The principle can be varyingly expressed in Latin phrases such as Nullum crimen, nulla poena sine praevia lege poenali (No crime can be committed, nor punishment imposed without a pre-existing penal law), nulla poena sine lege (no penalty without law) and nullum crimen sine lege (no crime without law).