What Is Customary International Law?
Alongside treaties, customary international law is one of the main sources of international law. Unlike treaties, which are written agreements, customary law develops from the consistent practices of states that are followed out of a sense of legal obligation.
Two elements are usually required for a rule to become customary international law:
State practice – many states must regularly act in a particular way.
Opinio juris – states must believe they are legally required to act in that way, not just doing so out of convenience or habit.
An example is the principle of diplomatic immunity. Even before it was codified in treaties, states widely accepted that diplomats should not be prosecuted under the host country’s laws. Over time, this practice, combined with the belief that it had legal force, became part of customary international law.
Customary law is important because it applies to all states, even those that are not party to specific treaties, as long as they have not consistently objected during its development. This ensures that certain fundamental principles are widely respected and enforceable.
By recognising customary international law, the international community has a way of formalising long-standing practices and promoting stability in international relations. It shows how international law can grow organically, reflecting both tradition and the shared expectations of states.