Criminal law, Body of law that defines criminal offenses, regulates the apprehension, charging, and trial of suspected offenders, and fixes punishment for convicted persons. Substantive criminal law defines particular crimes, and procedural law establishes rules for the prosecution of crime. In the U.S., substantive criminal law originated for the most part in common law, which was later codified in federal and state statutes. Modern criminal law has been affected considerably by the social sciences, especially in the areas of sentencing, legal research, legislation, and rehabilitation. See also criminology.

Legality

The principle of legality is recognized in almost all legal systems throughout the world as the keystone of criminal law. It is employed in four senses. The first is that there can be no crime without a rule of law; thus, immoral or antisocial conduct not forbidden and punished by law is not criminal. The law may be customary, as in some common-law countries; in most countries, however, the only source of criminal law is a statute (nullum crimen sine lege, “no crime without a law”).

Second, the principle of legality directs that criminal statutes be interpreted strictly and that they not be applied by analogical extension. If a criminal statute is ambiguous in its meaning or application, it is often given a narrow interpretation favorable to the accused. This does not mean that the law must be interpreted literally if to do so would defeat the clear purpose of the statute. The Model Penal Code incorporates a provision that was enacted in some U.S. state laws. The code recommends that its provisions be construed “according to the fair import of their terms,” which comes closer to the European practice.

Third, the principle of legality forbids the application of the law retroactively. A law must have been in effect at the time the act was committed so that a person may be convicted. This aspect of the principle is embodied in the ex post facto provisions of the U.S. Constitution and such international treaties as the European Convention for the Protection of Human Rights and Fundamental Freedoms (adopted 1950) and the International Covenant on Civil and Political Rights (entered into force 1976). It is also embodied in the Rome Statute creating the International Criminal Court (ICC; ratified 2002).

Fourth, the language of criminal statutes must be as clear and unambiguous as possible in order to provide fair warning to the potential lawbreaker. In some countries statutes may even be considered inapplicable if they are vague.