This thesis addresses the question of when the European Union is internationally responsible. More precisely, it examines the extent to which the European Union and its Member States bear responsibility for the violations of an international agreement committed as a consequence of the implementation of EU Law. The specific features of the multilevel system implementation of EU Law poses a series of very interesting questions as regards the EU’s relations with its Member States and their responsibility under International Law. The EU primarily implements its law through its Member States’ authorities. As such, should the EU bear responsibility for a violation committed by a Member State organ because it was implementing EU Law? Should a Member State which, in complying with a piece of EU legislation, violated an international obligation be held liable? This thesis examines these questions from a practical perspective. It examines how international bodies approach the issue. In this regard, the thesis sets out to examine whether there is a common thread in the way international courts and tribunals deal with the EU’s international responsibility. The thesis is structured in four parts. Part I identifies the basic issues surrounding the responsibility of the EU and its Member States under international law, by examining the context and the main issues both at the international level and domestic level. Part II of the thesis focuses on the main mechanism used by the EU to deal with its international responsibility. More specifically it addresses the question of whether the division of competence is a good way to establish the EU’s responsibility under an international agreement. Part III of the thesis continues with a practical examination of the EU’s responsibility. It examines how international courts and tribunals establish the EU‘s international responsibility in absence of any declaration limiting its competence. Part IV summarizes the conclusions of Parts I, II and III and contains a series of concluding remarks on the issue of the EU’s international responsibility and future developments.