International law, as a primitive legal system, with no formal method of making or enforcing laws, traditionally has relied heavily on customary international law, although increasingly the international community relies upon treaties as custom is slow to react to change. But custom remains crucial to the development of a comprehensive international legal system, as it is only rules of customary international law, which bind all states.

Custom in international law is a practice followed by states because they feel legally obliged to behave in such a way. Custom must be distinguished from behaviour that is motivated by reasons other than legal obligation such as courtesy, friendship or convenience.

A rule of customary international law derives its legal character through the possession of two elements: a- A material element. The material element refers to the behaviour and practice of states, which includes treaties, diplomatic correspondence, official legal statements, etc ….

For a rule to be considered part of customary international law, it must be generally adopted in the practice of states. That practice must be adopted by the majority of states, in any case, the majority of interested states. State practice must be constant and uniform. Finally, no minimum time limit on the duration of state practice is required before the rule may be considered part of customary international law, as the test is whether the rule is generally accepted.

The psychological element, although difficult to prove, is an essential element in the formation of customary law. It is not enough for the formation of customary law that there is general, uniform and consistent state practice. In order that this practice constitutes law, states must recognize it as binding upon them as law. State practice must be accompanied by a belief that the practice is obligatory, rather than merely convenient or habitual. This belief in the obligatory nature of the practice is called opinio juris.