The ICTY Erdemovic case demonstrated the practical relevance of duress in international criminal law. Taking this case as a starting point, this study examines whether and under which circumstances duress can constitute a defence in cases of unlawful killing. It consists of an in-depth analysis of the methodological and substantive questions raised by the Erdemovic case, in particular with regard to the sources of international criminal law and the specific requirements of duress. The study also examines Article 31(1)(d) of the Rome Statute of the ICC, discusses the controversial aspects of this provision, such as the subjective proportionality element, and goes on to propose an alternative wording. This book highlights the differences in the approaches of the ICTY and the ICC regarding duress and discusses whether a distinction between justifications and excuses would be necessary in international criminal law.