In 2005, the UN General Assembly unanimously adopted the doctrine of “the responsibility to protect” (R2P), requiring governments to safeguard their citizens against mass atrocities and authorizing the international community to act if they do not. In theory, R2P redefines long-standing norms of sovereignty and nonintervention. In practice, however, R2P remains an elusive goal, as revealed by this collection of essays, the best account yet of the philosophical and practical difficulties that bedevil R2P. Gareth Evans, one of the doctrine’s intellectual architects, contributes an incisive account of the origins and rise of the R2P idea and argues that despite setbacks, it is slowly emerging as a guiding principle. Edward Newman contends that the legitimacy of R2P ultimately hinges on the procedures used to authorize international action. Jean-Marc Coicaud looks at R2P through the lens of the recent crises in Gaza, Syria, and Ukraine, showing how the norm can be hijacked for cynical purposes. And Jonathan Graubart offers the most far-reaching critique, arguing that R2P merely provides great powers with a new justification for pursuing traditional military interventions.