Those designing and implementing the statutes of the various modern international and internationalized criminal courts and tribunals have adopted the principle of legality as a core principle. The way in which this has been done has, however, varied among the courts.

All of the international and internationalized criminal courts have accepted the rule of nullum crimen sine lege (nothing is a crime without [preexisting] law) in one way or another. The ad hoc UN Tribunals for the Former Yugoslavia and for Rwanda (ICTY and ICTR, respectively) and the Special Court for Sierra Leone (SCSL) have done so without a formal provision in their statutes. The Rome Statute of the International Criminal Court (ICC) has several provisions implementing the principle. Three of the internationalized courts (Kosovo, East Timor, and now Cambodia) adopted the principle by reference to major international documents on human rights, and the East Timor court incorporated it explicitly into the law. The Iraqi Special Tribunal incorporated the principle by reference to applicable Iraqi domestic law.

The principle of nulla poena sine lege (no penalty may be imposed without [preexisting] law) has been slightly more problematic. Some theorists have stated that nulla poena does not truly apply to international criminal law.