While the EU has championed "effective multilateralism" and experienced a dramatic internal reform process to improve its performance in external relations, broader multilateral processes have also undergone dramatic change. This handbook book addresses the increasingly contested issue of profound political importance: Europe's presence in multilateral institutions. It assesses both the evolving role of Europe in international institutions, and the transformations in international institutions themselves. Acknowledging that the category of international institutions comprises a highly diverse field of multilateral engagements this handbook presents a state of the art approach and is divided into seven parts: Part I examines the EU's diplomatic and legal personality in international relations and constitutes the internal foundation for the EU's engagement with international institutions. Part II assesses how EU multilateralism intersects with other international institutions and provides a means to assess the performance of international institutions as well as the EU itself in multilateral processes. Part III focuses on key institutions within the general UN system, such as the UN Security Council and the Human Rights Council as well as specific policy domains. Part IV focuses on the EU's engagement in a broad spectrum of issue-specific international agreements and international regimes, addressing issue areas such as non-proliferation of WMD, the emerging Gx-system (G7, G8, G20 etc), human rights and information technology. Part V focuses on relations with international economic organizations addressing the IMF, the World Bank, trade, the International Telecommunication Union, the OECD and environmental institutions. Part VI focuses on security organizations, including EU-NATO Relations, the OSCE, Global Non-Proliferation Institutions and the Private Military and Security Industry. Part VII examines the different ways in which the EU has been influenced by a range of global factors. This comprehensive volume brings together scholars and practioners to summarize and synthesize existing knowledge in the field, it will be of great interest to students and scholars of European politics, the EU's external relations, international relations, international organisations and international political economy.