Introduction The Lisbon Treaty emphasises the European Union’s (EU or Union) commitment to multilateralism, stating that it ‘shall seek to develop relations and build partnerships with . . . international, regional or global organisations’ and to ‘promote multilateral solutions to common problems, in particular in the framework of the United Nations’ (Art. 21(1), second para., TEU). Moreover, one of the objectives of the EU’s external action is to ‘promote an international system based on stronger multilateral cooperation and good global governance’ (Art. 21(2)(h) TEU). The EU being itself one of the most advanced forms of multilateral cooperation among states, it is quite understandable that it would be committed to ‘a stronger international society, well-functioning international institutions and a rule-based international order’ (European Council 2003a). The Union not only seeks to participate in this multilateral system, it actually aims to have a leading role in shaping it (European Council 2008). A key part of the EU’s engagement in this multilateral order is its participation in international organisations (IOs). The present chapter discusses the obstacles facing the EU when it seeks to assume a more prominent role in IOs. Such an effort is in line with the post-Lisbon external relations architecture of the EU, and its outcome may at times impact upon the diplomatic influence of the Union abroad. The first section of the chapter briefly outlines the different forms of participation the EU enjoys in various organisations. The next section examines the internal legal issues that still plague the Union when it seeks to be represented at IOs. Although the Lisbon Treaty was designed to eliminate many of these internal legal problems in EU external relations, new problems have emerged and some older ones are yet to be fully resolved. These include issues such as representation in areas of shared competences as well as the representation by the EU member states in IOs where the EU does not have any status. The chapter then turns to the external environment, an international legal system that is still heavily biased towards participation by states. Of the many international organisations that now play an important role in global governance, how does the EU decide which ones it shall develop closer cooperation with and the appropriate level of its representation in those bodies?