This book seeks to understand how and why we should hold leaders responsible for the collective mass atrocities that are committed in times of conflict. It attempts to untangle the debates on modes of liability in international criminal law (ICL) that have become truly complex over the last twenty years, and to provide a way to identify the most appropriate model for leadership liability. A unique comparative theory of ICL is offered, which clarifies the way in which ICL develops as a patchwork of different domestic criminal law notions. This theory forms the basis for the comparison of some influential domestic criminal law systems, with a view to understanding the policy and cultural reasons for their differences. There is a particular focus on the background of the German law which has influenced the International Criminal Court so much recently. This helps to understand, and seek a solution to, the current impasses in the debates on which model of liability should be applied. An entire chapter of the book is devoted to considering why leaders should be held responsible for crimes committed by their subordinates, from legal, moral and pragmatic perspectives. The moral responsibility of leaders is translated into criminal liability, and the different domestic models of liability are translated to the international context, in such a way as to appeal to advanced students of ICL, academics, and practitioners who want to understand the complexities of leadership liability in international criminal law today and identify the best way to approach it. Cassandra Steer is Executive Director of Women in International Security Canada, and Junior Wainwright Fellow at McGill University, Canada. She holds a Ph.D. in Law from the University of Amsterdam, The Netherlands.