Summary

Custom has been one of the main sources of international law through times. International custom is a practice which is legally binding on its followers. Custom is formed based on the following: first, continuous and uninterrupted activity (or inactivity) of the state, and second, a particular conduct must be required or permitted by international law. Custom may be regional or universal. After WW II, custom gave way to several new forms of behaviour which are regulated differently. This was caused by an increasing number of states participating in international relations and the rise of socialist states. Nevertheless, there are a number of rules of common law which are accepted by states which have been created recently. Custom has retained its great importance at the level of economic interests and in regulation of international relations.
The article provides an overview of different conditions necessary for the development of custom as suggested by theorists, introduces the substantive sources of custom and brings out positions on the role of contracts in the formation of custom.