This chapter undertakes a critical legal examination of R2P’s Pillar 1 obligation for host States to protect their populations from mass atrocity crimes. It analyses the definition and legally prohibited status of each of the four crimes of genocide, war crimes, crimes against humanity and ethnic cleansing, drawing upon relevant treaty law, customary international law, and international case law. This analysis is complemented with an overview of opinio juris with respect to R2P’s Pillar 1 to reflect upon this Pillar’s acceptance by States as well as its normative gaps.