The diet of Paranthropus boisei has long been a subject of discussion and debate. The observation that eastern African P. boisei consumed primarily C4 foods (tropical grassesGrass or sedgesSedges) implies a diet distinct from that of extant apes and most other early hominins, and necessitates reevaluation of its morphologyMorphology and dietary ecologyEcology. In this chapter, we explore the hypothesis that the masticatory apparatusMasticatory apparatus of P. boisei is indicative of grassGrass leaf consumption. Given the lack of suitable primate analogs, we focus on comparisons with extant large mammalianlarge mammalian grazers. Several morphological features typical of P. boisei, including its massive postcanine teethTeeth, with molarized premolarsPremolar and thick enamel, exaggerated masticatory musculature, and robust mandibles are consistent with the grazingGrazing hypothesis. The lack of shearing crests on the occlusal surfaces of its postcanine teethTeeth, which is unexpected for a grazer, may be due to evolutionary constraints, and the resulting relatively inefficient comminutionComminution of grassesGrass could explain why the masticatory anatomy of P. boisei is so unusual compared with extant primates.