International law is a collection of developing rules which governs the relationship between nations.  The range of subjects directly concerned with international law has widened considerably, moving beyond the issues of war, peace, and diplomacy to include human rights, trade issues, space law, and international organizations.

Article 38 (1) of the International Court of Justice identifies three sources of international law: treaties, customary international law, and general principles of international law (jus cogens)

Customary international law is a primary source of international law and is derived from customs. For example, the law of war was long a matter of customary law before it was codified in the Hague Conventions of 1899 and 1907, Geneva Conventions, and other treaties.  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 Statute of the International Court of Justice acknowledges the existence of customary international law in Article 38(1)(b), incorporated into the United Nations Charter by Article 92.  The article states that the Court shall apply international custom while deciding disputes.

Customary international law consists of rules of law derived from the consistent conduct of States.  The elements of customary law are as follows:

• Customary international law can be discerned by a widespread repetition of similar international acts over time by States (states 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.

A marker of customary international law is consensus among states exhibited both by widespread conduct and a discernible sense of obligation.