Customary international law

Customary international law are those aspects of international law that study the principle of custom. Along with general principles of law and treaties, custom is considered by the International Court of Justice, jurists, the United Nations, and its member states to be among the primary sources of international law.

The vast majority of the world's governments accept in principle the existence of customary international law, although there are many differing opinions as to what rules are contained in it.

The International Court of Justice Statute defines customary international law in Article 38(1)(b) as "evidence of a general practice accepted as law." This is generally determined through two factors: the general practice of states and what states have accepted as law.

There are several different kinds of customary international laws recognized by states. Some customary international laws rise to the level of jus cogens through acceptance by the international community as non-derogable rights, while other customary international law may simply be followed by a small group of states. States are typically bound by customary international law regardless of whether the states have codified these laws domestically or through treaties.

See jus cogens.

A peremptory norm (also called jus cogens, Latin for "compelling law") is a fundamental principle of international law which is accepted by the international community of states as a norm from which no derogation is ever permitted. These norms rooted from Natural Law principles, and any laws conflicting with it should be considered null and void. Examples include various international crimes; a state which carries out or permits slavery, torture, genocide, war of aggression, or crimes against humanity is always violating customary international law.

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