Quis custodiet ipsos custodies? In other words, who guards the guardians? At a time when the mandate of many peace support operations includes halting violations of international humanitarian law by third parties, there is still a lack of clarity concerning accountability of peace support operations themselves. This book addresses that accountability, focusing on peace support operations under the command and control of the United Nations and the North Atlantic Treaty Organization. It is concerned with the accountability of international organizations as well as troops contributing and member states, but not of individuals. Drawing on existing and emerging doctrines of international law, including the law of state responsibility, the law of responsibility of international organizations, international institutional law and international humanitarian law, and on the basis of state practice, this book makes a strong plea for improving mechanisms to implement the accountability of peace support operations under international humanitarian law. The Paul Reuter Prize 2006 was awarded to Marten Zwanenburg for this book.