The aim of this book is twofold: to take a multi- and interdisciplinary approach to extreme forms of collective violence and to provide university students and all other interested scholars and practitioners within the field with an integrated textbook. After an explanation of different types of international crimes the book will focus on two of the three main actors: the perpetrator and the bystander. The other actor – the victim – is dealt with in relation to the first two actors. Although the victim is very important, the victim has no central place in this study. The focus on the role of perpetrators and bystanders and our efforts to explain and to enlighten their behaviour is in no way meant to excuse their acts. An explanation and elaboration of the roles and positions of perpetrators and bystanders is in our view the best contribution we can offer to clarify the processes which have led to these international crimes and other gross human rights violations. With our book we hope to contribute to the prevention of these atrocities. In revealing the underlying factors from a very broad and multi-disciplinary approach, we aim to contribute to a more humane future