Compared with municipal law the various methods by which rights and duties may be created in international law are relatively unsophisticated. Within a state, legal interests may be established by contracts between two or more persons, or by agreements under seal, or under the developed system for transferring property, or indeed by virtue of legislation or judicial decisions. International law is more limited as far as the mechanisms for the creation of new rules are concerned. Custom relies upon a measure of state practice supported by opinio juris and is usually, although not invariably, an evolving and timely process. Treaties, on the other hand, are a more direct and formal method of international law creation.
States transact a vast amount of work by using the device of the treaty, in circumstances which underline the paucity of international law procedures when compared with the many ways in which a person within a state's internal order may set up binding rights and obligations. For instance, wars will be terminated, disputes settled, territory acquired, special interests determined, alliances established and international organisations created, all by means of treaties. No simpler method of reflecting the agreed objectives of states really exists and the international convention has to suffice both for straightforward bilateral agreements and complicated multilateral expressions of opinions. Thus, the concept of the treaty and how it operates becomes of paramount importance to the evolution of international law.
A treaty is basically an agreement between parties on the international scene. Although treaties may be concluded, or made, between states and international organisations, they are primarily concerned with relations between states. An International Convention on the Law of Treaties was signed in 1969 and came into force in 1980, while a Convention on Treaties between States and International Organizations was signed in 1986. The emphasis, however, will be on the appropriate rules which have emerged as between states. The 1969 Vienna Convention on the Law of Treaties partly reflects customary law and constitutes the basic framework for any discussion of the nature and characteristics of treaties. Certain provisions of the Convention may be regarded as reflective of customary international law, such as the rules on interpretation, material breach and fundamental change of circumstances. Others may not be so regarded, and constitute principles binding only upon state parties.