The tragic events in the 1990s in Rwanda, Srebrenica and Kosovo, and the crisis in Libya in 2011 have triggered a fundamental rethinking of the role and responsibility of the international community. It is now accepted that while individual states continue to bear the primary responsibility to protect their populations against genocide, ethnic cleansing, crimes against humanity and war crimes within their boundaries, the international community should step in when the state is unable or unwilling to provide such protection. The principle of the Responsibility to Protect, or RtoP, reflects this recognition, and provides the normative basis for involvement of the international community in cases of mass atrocities. This thoughtful work is a major contribution towards clarifying what RtoP can offer, moving from principle to practice. It spans the disciplines of international law, international relations, and moral philosophy.