In insular Southeast Asia Presbytis and Trachypithecus langurs show a complex distribution pattern whereby in certain areas only one species is present, in other areas two or three species of the same genus live sympatrically, and in yet other areas two, three or even four species of the two genera co-exist. Given their morphological similarity it is intriguing to unravel the ecological differences that allow these species to live side by side. Presbytis langurs are more confined to wet forests than Trachypithecus langurs; their range includes drier regions, high mountains and small islands. In terms of diet (young, old leaves, fruits, seeds) or activity budgets there is a large variation within species and no clear difference between co-occuring species emerges. Home range sizes for Presbytis langurs (~35 ha) is larger than those for Trachypithecus langurs (~20 ha), but day ranges appear to be more similar (~500 to 1,500 m). The variation in group sizes and composition (especially the number of adult males) differs in that Presbytis langurs typically live in smaller groups (~5 to 12 individuals), mostly with one adult male, than Trachypithecus langurs groups (frequently over 20 individuals) that frequently have two or more adult males present.