Scholars from different disciplinary backgrounds have studied why perpetrators of international crimes commit these horrendous acts. Initially, historians and psychologists focused on this debate, which was heavily centred on the Second World War. Over the years, scholars with more diverse disciplinary backgrounds, studying a wide array of cases, using both qualitative and quantitative research methods, began to investigate perpetrators of international crimes and terrorism. Recently, this multi- and interdisciplinary debate has become known as perpetrator studies. This is the first book to take stock of the state of the art of this new field of study. It analyses the most prominent theories, methods, and evidence to determine what we know and what we think we know about perpetrators, as well as the ethical implications of gathering this knowledge. It traces the development of perpetrator studies while pushing the boundaries of the field by including contributions from authors from a wide array of disciplines, including criminology, history, law, sociology, psychology, political science, religious studies, and anthropology. Authors cover numerous case studies, including prominent ones such as Nazi Germany, Rwanda, and the former Yugoslavia, but also those that are relatively under-researched and more recent, such as Sri Lanka and the Islamic State, and use various research methods, including but not limited to, trial observations and interviews.