The European Union is one of the world's biggest economies. However, its role as an international actor is ambiguous and it's not always able to transform its political power into effective external policies. The development of an 'assertive' European Union challenges the image of an internal project aimed at economic integration and international relations theories based on unitary state actors. This book systematically links the EU's external relations to existing political theories, showing how existing theories need to be modified in order to deal with specific characteristics of the EU as an international actor.