The Responsibility to Protect (R2P) is an international concept that aims to protect civilians from mass atrocities. In a short period of time, the R2P concept developed from a philosophical theory on sovereignty into a new norm in international relations. This chapter considers the gradual acceptance of the concept at the United Nations, where it has brought about a paradigm change in thinking and language regarding prevention, protection, intervention and rebuilding in situations of possible mass atrocities. We examine the substance and stages of development of the R2P concept. A conceptual breakthrough led to a diplomatic breakthrough because of the power of ideas and determined leadership by key players in delegations, the UN Secretariat and NGOs. While the concept now enjoys wide acceptance, its implementation still remains uneven. In this chapter we examine how an idea can break through to the status of an international norm.