How is the international responsibility of the European Union determined? In the context of the multilayered and ever evolving Union legal order, the Lisbon Treaty has introduced considerable changes to the Union's participation in international affairs. These have rendered this thorny question an even more pressing concern not only for the European Union and its Member States but also for third countries and international organisations. Based on papers delivered at the bi-annual EU/International Law Forum organised by the University of Bristol in May 2011, this volume brings together EU and international law experts to address the various questions raised by the Union's international responsibility. It discusses horizontal issues, such as the concept of responsibility of international organisations in the evolving international legal order and the different techniques available for determining responsibility. It also focuses on specific policy areas (trade, investment, environment, security and defence, human rights) by approaching them from both an EU and international law perspective.