When acceding to the European Union, Member States retained their capacity to conclude treaties under international law. Yet, EU law imposes significant restraints on that freedom. This thesis examines how, and to what extent, EU law constrains the treaty-making powers of the Member States. The manuscript is organised in three parts. Part I analyses constraints arising in areas of exclusive Union competence, distinguishing between a priori exclusivity, ERTA exclusivity and Opinion 1/76 exclusivity. Part II examines restraints operating within areas of non-exclusive competence, covering shared, parallel and supplementary competences. Building on these findings, Part III addresses cross-cutting issues. It reflects upon the relevance of the Lisbon competence catalogue, explores the extent of the Member States’ exclusive competences, assesses the permissibility of collective actions of the Member States and examines the judicial enforcement of restraints imposed by EU law. This is followed by conclusions and recommendations.