Measurements were taken on the upper and lower molars of 37 species of primates and one tupaiid to assess the relative importance of shearing, crushing and grinding features. Significant correlations were found between pairs of allometrically standardized dimensions which measure the same molar function (shearing, crushing, or grinding). Correlations between pairs of dimensions which do not measure the same function are not significant. Second molar adaptations for shearing, crushing, and grinding, as well as the length of the second lower molar, and the total surface of the post-canine dentition are negatively allometric with respect to metabolic rate. Species which take different proportions of fruit, leaves, and insects in their diets have different molar structure. Frugivores have small teeth for their adult body size with poorly developed shearing, crushing, and grinding features on their molars. By contrast, leaf-eating species tend to have large teeth for their adult body size with well developed shearing, crushing, and grinding. The second molars of insectivorous species were found to parallel closely those of leaf-eating species. The two groups are clearly distinguishable from the former on the basis of body size alone: the smallest living primate leaf-eater is on order of magnitude larger than the largest living primate insectivore.