The colobine monkey genus Semnopithecus includes gray, Nilgiri, and purple-faced langurs. Gray langurs (traditionally, S. entellus) have among the most extensive geographical ranges of all nonhuman primates, from Sri Lanka to the Himalayas, and are historically the most frequently studied colobine. Gray langurs have become key models in studies of sexual selection, niche partitioning, socioecology, optimal foraging theory, and movement ecology. Nilgiri langurs (S. johnii) are found in the Western Ghats of southern India; in addition to foundational descriptive studies, research on Nilgiri langurs has included inspired work on nutritional ecology and behavioral rhythms. Purple-faced langurs (S. vetulus) are found in varied Sri Lankan habitats and are perhaps the least-studied members of the genus; notwithstanding, they have been the subject of influential, interdisciplinary studies of locomotion, as well as one of the few detailed studies of predation on a colobine. All Semnopithecus appear to be characterized by considerable phenotypic plasticity, including a fluid leaf and/or fruit-centered diet with considerable site-specific differentiation in supplemental food categories, and variable social arrangements including single-male and/or multi-male, multi-female groups. There remain many opportunities to further advance our knowledge of this genus from novel biological, psychological, and anthropological perspectives.