This book shines light on the role of ‘de facto international prosecutors’ as an emerging phenomenon. They are ‘private’ non-state actors, and state legal ‘officials’ in a foreign court, which pursue criminal accountability for those most responsible for core international crimes. It explains how and why they adopt the tasks of the offices of international prosecutors when local options to investigate fail and no international criminal tribunal is available. A particular focus of the book are the roles of witnesses and victims of core international crimes as ‘de facto international prosecutors’. The study reveals how crimes, such as torture, which have evolved as jus cogens and erga omnes obligations enable them to pursue prosecution in foreign courts exercising universal jurisdiction. The paper proposes two theoretical frameworks: 1) to conceptualise ‘de facto international prosecutors’; 2) to explain how ‘de facto international prosecutors’ conceptualise international criminal law.