Nullum crimen sine lege ("no crime without law") is the moral principle in criminal law and international criminal law that a person cannot or should not face criminal punishment except for an act that was criminalized by law before he/she performed the act. Subtler versions of this principle require crimes to be declared in unambiguous statutory text.
Some scholars make a distinction between substantive and procedural law in the context of nullum crimen sine lege. On this view, a change in substantive law leading to liability must occur before the act is committed, but a change in procedural law leading to liability may occur after the act is committed. For example, extending the statute of limitations to allow the prosecution of crimes that occurred far in the past would constitute a change in procedural law.
The notion of nullum crimen sine lege has been relatively important in the context of international criminal law. The acts punished by the international criminal tribunals were of ambiguous legality in the conflict or atrocity situations in which they were committed.
Nullum crimen sine lege is sometimes called the legality principle. Another formulation is "nullum poena sine lege" ("no punishment without law").
 George Fletcher. Basic Concepts of Criminal Law (1998), ch. 1.