The history of genocide prevention across the long twentieth century is a history of failure. Only very rarely have international actors acted collectively to halt or mitigate a genocide underway or one in the making. That said, across the arc of the twentieth century, there have been important developments and milestones in norms, law and policy as they pertain to prevention. These changes amount to incremental, if episodic, progress. They do not, however, amount either to a consistent, effective multilateral policy or to consistent, effective foreign policies of particular states. But if we measure the global norms, legal instruments, institutions, and policy tools available at the start of the twentieth century compared to those at the start of the twenty-first century, there are significant differences to observe. This chapter summarises those changes and charts them across key cases. It concludes with a discussion of future challenges and opportunities for genocide prevention.1