This book provides a critical introduction to the core elements of international criminal law. It does so by provoking thought on what international criminal law is, or could be, by contrasting the practice of widely recognised state-based actors and institutions such as the International Criminal Court with practices associated with non-state actors in particular citizens' tribunals. International criminal law is now established as an essential legal and institutional response to atrocity. However, it faces a series of political and practical challenges. It is vital to consider its limits and potential, as well as the ways and extent to which those limitations might be addressed. Many actors with very different visions of its nature and parameters play a role in shaping the meaning of international criminal law whether that be in official or unofficial spaces. This book explores the principles and institutions of international criminal law alongside the alternative visions of it put forward by citizens' tribunals. In so doing it encourages reflection on that law's multiple meanings and usages in order to provoke consideration of what it means, and might mean, to deploy international criminal law today.