In contrast to the practical focus of Part II of this book, Part I, composed of this chapter, proposes a theoretical framework for how de facto international prosecutors extend the reaches of international criminal law. Part II provides the empirical account of how and why de facto international prosecutors, particularly witnesses and victims of core international crimes, adopt the practical tasks required to hold a senior leader suspected of core crimes accountable in foreign or international courts. The theoretical framework outlined in this chapter is an attempt to answer the following questions: what dynamics and mechanisms can we observe and theorise as effective or not (beyond these actors’ own legal interpretive work) in pursuing accountability? How (or when) do particular interpretations of international law ‘win out’ over others in court judgements? The chapter also serves to further define and conceptualise de facto international prosecutors and the international legal order within which they operate.