The odd-nosed monkeys represent a monophyletic group of phenotypically unique primates. They include five species of snub-nosed monkeys (Rhinopithecus), three species of doucs (Pygathrix) as well as simakobu (Simias concolor) and the proboscis monkey (Nasalis larvatus). These species are ecologically diverse and inhabit a gradient of environments in China and southeast Asia, from wet equatorial forests to alpine forests. Odd-nosed monkeys tend to be shy and difficult to habituate, and often range over relatively large areas. Many are also renowned for inhabiting remote areas with rugged terrain and inclement climatic conditions. It is thus not surprising that, until fairly recently, they were among the least studied and most enigmatic extant diurnal primates. However, since the beginning of the new millennium, an expanse of new and exciting research has been conducted on these colobines. In this chapter, we provide an overview of the current knowledge of the ecology and behaviour of odd-nosed monkeys and analyse some of their key behavioural traits and ecological adaptations in light of what is known about other colobines and primates as a whole.