Customary international law is an aspect of international law involving 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.
Many governments accept in principle the existence of customary international law, although there are differing opinions as to what rules are contained in it.
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.