Featuring an outstanding international line-up of contributors, this edited volume offers a timely examination of two of the most crucial and controversial issues in international relations, namely the evolution of particular concepts of multilateralism and whether international security institutions are the objects of state choice and/or consequential. The book combines a variety of theoretical perspectives with detailed empirical examples. The subjects covered include: The development and contemporary application of the concept of multilateralism; American foreign and security policy in the post 9/11 era (unilateralism vs. multilateralism); Humanitarian intervention and liberal peace; Case studies of a variety of security institutions including the EU, UN and NATO; and, a broad selection of geographical examples from North America, Europe and Asia. This book is a significant contribution to the contemporary debate on multilateralism and the effects of multilateral security institutions and will be of great interest to scholars of international relations and security studies.