The ecology and behavior of the four species of Rhinopithecus, snub-nosed monkeys, are rapidly becoming well known. New field studies reveal in depth the striking adaptations of these colobines. Diets range from those typical for tropical colobines to diets dominated by lichens. The monkeys form bands, at times consisting of more than 400 individuals; these bands are based on the one-male, multi-female units common in colobines. We review the diet, range use, and social organization of snub-nosed monkeys, and then explore the power of socioecological theory to explain their multilevel social organization.