Customary international law

Customary international law consists of international legal obligations arising from established or usual international practices, which are less formal customary expectations of behavior often unwritten as opposed to formal written treaties or conventions. Customary law is also referred to as opinio juris ("opinion of law"), which is a term used in international law to signify state practices taken under the belief that such actions are legal obligations. Generally, customary international law applies equally to all states. 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.

Rules that are considered customary law are binding upon all states. Customary international law need not be codified in a treaty. If a treaty, or any portion thereof, becomes customary law, it will bind all states that are not persistent objectors. A treaty or international agreement can bind a state not party to the treaty when: the treaty codifies customary international law; or when the treaty has become customary law or represents an obligation erga omnes. For example, the Vienna Convention on the Law of Treaties is widely described as codifying customary international law concerning treaties.

Many governments accept in principle the existence of customary international law, although there are differing opinions as to what rules are contained in it. A rule becomes customary international law if two requirements are met: (1) There is a state practice that "appears to be sufficiently widespread, representative as well as consistent" showing that a significant number of states have used and relied on the rule in question and the concept has not been rejected by a significant number of states, and (2) states are motivated by a belief that they are legally compelled to accept the legitimacy of the rule in question because a rule of customary law obligates them to do so (opinio juris).

In 1950, the International Law Commission listed the following sources as forms of evidence of customary international law: treaties, decisions of national and international courts, national legislation, opinions of national legal advisors, diplomatic correspondence, and practice of international organizations. In 2018, the Commission adopted Conclusions on Identification of Customary International Law with commentaries. The United Nations General Assembly welcomed the Conclusions and encouraged their widest possible dissemination. However, such rules are typically less definite in their formulation, and thus subject to doubt in practice.