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Cat Ba langurs (Trachypithecus poliocephalus), endemic to Cat Ba Island (Vietnam), are a member of the limestone langur group (francoisi species-group) found in Laos, Vietnam, and China. With less than 75 individuals in the world, these Cat Ba langurs are Critically Endangered. This dissertation represents the first long-term behavioural study of the species (549 contact hours across 11 months), and provides socioecological information for two groups (n=17-20) living on the Cua Dong fjord, which can be used in conservation management. Like most other colobines, the majority of the Cat Ba langur diet is leaves (84% of their dietary budget). This may explain their activity budget, which is primarily inactivity (55% of their activity budget), followed by foraging (19%) and social behaviour and locomotion (12% each). Activity and dietary budgets vary seasonally, with animals spending more time in social behaviours in the wet season, when they eat more fruit, and more time in foraging in the dry season, when leaves are ingested more, suggesting they are energy maximisers. In addition, age differences were found, with adults spending the most time in social behaviours and resting. Social behaviour primarily includes grooming and playing with others - play is more common in younger animals, while older animals tend to groom more. Overall, they spend 58% of their days not in proximity to any other langurs. Adult males spend the most time alone, and seem to avoid young langurs. Disputes tend to be between adult females, and two females only tend to come together if there is a young langur acting as a ‘social glue’. Home ranges varied between groups with the larger group’s range being 50ha compared to just 22ha for the smaller group. In both areas, rocks and sparsely covered areas are used most often, which is due to the shrubby, discontinuous vegetation. Most of the langurs’ observable time is spent on exposed slopes (47%), followed by steep cliffs (38%), summits (11%), valleys (3%), and the ground (1%); this varied seasonally, likely to balance foraging needs with thermoregulation. The langurs used 22 sleeping sites, including ledges (61%) and caves (17%). When newborn, Cat Ba langurs are bright orange. They start to lose this natal coat as an infant, and become much darker during the young juvenile stage. As individuals age, they also become more independent and start to forage and locomote more on their own. These reported behaviours can be used to create a baseline for activity budgets, home range size, and habitat use, and development and maturation that can be used for comparative purposes in future studies. Results find that animals are behaviourally similar to other related species and their biggest threat is likely their small, fragmented population structure. To combat this, I support habitat protection, patrols, and enforcement; education and training; habitat corridors; and limiting the human population, as these support an entire ecosystem while teaching locals the importance of biodiversity, reducing resource competition, and fragmentation from infrastructure, and providing a means for the langurs themselves to disperse.