This book examines the concept of individual criminal responsibility for serious violations of international law, i.e., aggression, genocide, crimes against humanity, and war crimes. Such crimes are rarely committed by single individuals. Rather, international crimes generally connote a plurality of offenders, particularly in the execution of the crimes, which are often orchestrated and masterminded by individuals behind the scene of the crimes who can be termed ‘intellectual perpetrators’. For a determination of individual guilt and responsibility, a fair assessment of the mutual relationships between those persons is indispensable. By setting out how to understand and apply concepts such as joint criminal enterprise, superior responsibility, duress, and the defence of superior orders, this work provides a framework for that assessment. It does so by bringing to light the roots of these concepts, which lie not merely in earlier phases of development of international criminal law but also in domestic law and legal doctrine. The book also critically reflects on how criminal responsibility has been developed in the case law of international criminal tribunals and courts. It thus illuminates and analyses the rules on individual responsibility in international law.