Introduction In this chapter, I describe and analyse the roles played by the EU in modern multilateral negotiation. The main focus is on EU roles in institutions that mirror the emerging new world order, notably the World Trade Organization (WTO) and climate change conferences of the parties (COPs). Using role theory, I outline potential roles that actors may enact in multilateral negotiations, but also rely on concepts and instruments from negotiation theory. I argue that changing distributions of aggregate and issue-specific power create challenges for the Union and require a reconsideration of what roles – in terms of inter alia leadership, bridge-building and coalition-making – that are appropriate in these negotiation contexts. This is illustrated with analyses of the Doha Round and the COPs in Copenhagen, Cancun and Durban. In my analysis, I specifically highlight the EU’s reactions to and handling of the emerging powers (labelled the BASIC countries in climate change and BRIC countries in trade negotiations) but also its relations to the US and to the group of developing countries, organised in G77/90. The rise of the new economic powers constitutes one important part of the changing context for EU diplomacy and a challenge for the EU’s emerging diplomatic system. I start by suggesting a number of roles that state actors tend to play in international multilateral negotiations. In the following section, the changes in global aggregate and issue-specific power that have taken place since the end of the Cold War are outlined, with an emphasis on developments in trade and climate change. I then turn to the linkages between power transformation and role-playing as evidenced in actual multilateral diplomacy, beginning with an analysis of the EU’s role in the WTO trade negotiations, followed by an account of its role in global climate change negotiations. Both cases are structured by focusing on EU roles and role performance during three crucial multilateral meetings; one that took place before the rise of the emerging powers, two thereafter. I end by comparing the two cases and by drawing a number of general conclusions regarding trends in EU roles and role behaviour in the context of a new world order.