Treaties are only examined here in so far as they constitute a source of law. Treaties, as Article 38 provides, may be between two states (bilateral) or between several states (multilateral). However, all treaties involve a contractual obligation for the parties concerned, and consequently, create law only for the parties agreeing to the terms of the treaty. But the provisions of multilateral treaties may become customary international law, and thus creating general obligations.

Treaties are the most important source of international law.

Customary law and law made by treaty have equal authority as international law. Usually they are complementary but difficulties can arise if the treaty and customary law stipulate contradictory obligations. In those rare cases of conflict, and unless the parties have expressed otherwise, generally the later, be it custom or treaty, prevails as between the same parties.