The Proboscis monkey is a fascinating primate and it is found only on the tropical island of Borneo in Southeast Asia. The wild populations are often restricted to coastal rainforest, mangrove forest and riverine forest. They feed on the naturally-occurring leaves and fruits from the forest. Little is know about their population dynamics, ecology and social behavior. Besides, it is not all that easy to keep them alive in captivity due to their specialized leafy diet. Only a few zoos in Asia display these rare monkeys. The author, an international primatologist with three decades of field research experience, has studied these amazing monkeys, both in the wild (in-situ) and in captivity (ex-situ). In this monograph, the author has synthesized his scientific research on proboscis monkeys in the wild and in zoo setting with details on population, ecology, behavior, conservation and management. Therefore this monograph should serve as an introductory and reference text on primatology and animal behavior related courses in colleges, universities and institutes of higher learning globally.