The European Union has competence to enter into international-law relations and conclude international agreements. From a theoretical point of view, these agreements do not constitute a homogeneous source of EU and international law but di er based not only on a criterion of their content, but also who is their party. Some of these agreements have been concluded solely by the European Union, some together with its Member States and some by Member States on behalf of the Union. e article thus provides a classi cation of the international agreements that have been concluded by the European Union and in its second part it focuses on problems with interpretation of these agreements. Since both the European Union and Member States are involved, the crucial question is who is competent to determine the authoritative, correct, and binding interpretation of these agreements and to what extent.