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In addition to treaties and other expressed or ratified agreements that create international law, the International Court of Justice, jurists, the United Nations and its member states consider customary international law, coupled with general principles of law, to be primary sources of international law. The vast majority of the world's governments (including the United States) accept in principle the existence of customary international law, although there are many differing opinions as to what rules are contained in it.

The UN charter acknowledges the existence of customary international law (article 38(1)(b) of the Statute, incorporated into the Charter by article 92 thereof): "The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply... international custom, as evidence of a general practice accepted as law;".

Customary international law "... consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way." (Rosenne, Practice and Methods of International Law, p. 55). It follows that customary international law can be discerned by a "widespread repetition by States of similar international acts over time (State practice); Acts must occur out of sense of obligation (opinio juris); Acts must be taken by a significant number of States and not be rejected by a significant number of States."

Amnesty International writes that:

Customary international law results from a general and consistent practice of states followed out of a sense of legal obligation, so much so that it becomes custom. As such, it is not necessary for a country to sign a treaty for customary international law to apply.^{[citation needed]}

In other words, customary international law must be derived from a clear consensus among states, as exhibited both by widespread conduct and a discernible sense of obligation.

Customary international law can therefore not be declared by a majority of States for their own purposes; it can be discerned only through actual widespread practice. For example, laws of war were long a matter of customary law before they were codified in the Geneva Conventions and other treaties.

A particular category of customary international law, jus cogens refers to a principle of international law so fundamental that no state may opt out by way of treaty or otherwise. Examples of this is the act of genocide and crimes against humanity.

• Sources of International Law
• Universal jurisdiction
• Custom (law)