This chapter broadly covers what we know of the natural history of the olive colobus monkey, the only monotypic African colobine that shows a suite of unique traits. Distributed in the upper Guinea forests of West Africa, these cryptic, drab-coloured monkeys tend to form long-term and often permanent polyspecific associations with guenon groups as part of their predation avoidance strategy. They carry their infants in their mouths and show several morphological and behavioural traits (e.g., sexual swellings, canine dimorphism, extra-group copulations) that indicate that sexual selection has been important in their evolutionary history. They are selective feeders that favour young leaves and often eat lianas. No long-term studies are currently being conducted on olive colobus monkeys and there is much remaining to be discovered about their behaviour and ecology.