In the fifteen years since the publication of the report 'The Responsibility to Protect' by the International Commission on Intervention and State Sovereignty, there has been a burgeoning literature on all aspects of R2P. This review article focuses on five issues. First, it revisits the shift from 'humanitarian intervention' to R2P as the key innovation in 2001, highlighting the political, conceptual, normative, procedural and operational differences between the two. Second, it examines the state of knowledge regarding the causes of atrocities; the institutional vulnerabilities and points of resilience; the pathways from simmering animosities to mass killings; the indicators and precursors; and the most effective preventive and response mechanisms. Third, it reviews the unsatisfactory state of R2P implementation. Fourth, it discusses the main R2P actors, from international organizations and key groups of states to individuals. Finally, it addresses the continuing scepticism about R2P, in that it does not resolve all the dilemmas of how outsiders can provide timely, decisive and effective assistance to any group in need of protection.