This chapter explores the origin of transitional justice, its meaning and its historical development. It traces the genealogy of transitional justice from ancient times through to the contemporary grounding of transitional justice in the prosecutions at the International Military Tribunal at Nurnberg and the International Military Tribunal for the Far East. The chapter examines the challenges of defining transitional justice and articulates a definition of it for this book. It focuses on the concept of accountability, an important force in the development of transitional justice. Transitional justice measures can be discerned in the practices of societies in different epochs in history in various parts of the world. While many accounts of the development of transitional justice focus on post–World War II, there is a case to be made for tracing the origins of transitional justice to well before the 20th century.