In past decades the European Community and the European Union have concluded and become a party to a great number of bilateral and multilateral international agreements. In some cases the Community or the Union acts as the sole contracting party but in an increasing number of cases the Community acts as a joint contracting party alongside the Member States. The author analyses to what extent the Community and the Union apply or are bound by existing rules of international treaty law as embodied in the two Vienna Conventions and customary international law. His analysis is preceded by an extensive description of the Community and Union's external treaty-making powers. The study concludes with proposals regarding the manner in which the law of treaties should be amended in order to regulate more effectively the treaty relations of the European Community and the European Union, in particular in the case of mixed agreements.