This book offers a comprehensive analysis of the European Union as an International Actor and of its foreign policy of ‘effective multilateralism’ in both theory and practice. The core argument is that the EU has fostered integrative links - not only between states, but more recently also among international organisations. The study highlights the successes and critically examines the weaknesses of the EU’s effective multilateralism with NATO and the United Nations and offers concrete proposals for strengthening the EU as an inter-organisational security actor and Integrative Power in the short- and long-term.The first part of this study analyses major theories of multilateralism and takes stock of 40 years of scholarship on conceptualizing the EU as a distinct foreign policy actor. In this context, this book introduces the new concept of the ‘European Union as an Integrative Power’ as a core feature of the EU’s nature and impact in international affairs. The second part of the book examines the successes and drawbacks of the EU’s cooperation with NATO and the United Nations in the field of peace, security and international crisis management. With the help of detailed case studies of the EU’s CSDP operations in Macedonia, Bosnia and the Democratic Republic of Congo as well as the first detailed analysis of the EU’s relations with the Standby High Readiness Brigade for United Nations Operations (SHIRBRIG), this study provides a thorough assessment of the integrative as well as adverse impacts of the EU’s approach to EU-NATO and EU-UN ‘effective multilateralism’.This book will provide a valuable resource for graduate students, academics and practitioners interested in the EU as an International Actor, ‘effective multilateralism’ and the growing phenomenon of inter-organisational relations.Joachim A. Koops is Senior Associate Researcher at the Institute for European Studies (IES) and Assistant Professor of Political Science at Vesalius College, Free University of Brussels (VUB), where he is also the founding Academic Director of the European Peace and Security Studies (EPSS) Programme.