Concentrating on their presence in Borneo, the ecology and conservation of two large Southeast Asian primates, the orangutan Pongo pymaeus and the proboscis monkey Nasalis larvatus are reviewed. The former species occurs only in Borneo and Sumatra and the latter only in Borneo. The comparative threats facing these two endangered primates and their approximate numbers in the wild are put into perspective. The long-term survival of both species is adversely affected by the degradation and conversion of their suitable forest habitat by logging and agriculture, the occurrence of hunting, poaching and forest fires. The effectiveness of measures to conserve these species are discussed and evaluated from the standpoint of economics. It is concluded that informed assessment of the opportunity costs of conserving these species and their habitat is required and better incentives for law enforcement must be created. Properly regulated ecotourism may draw the necessary attention to the plight of these species and may even help fund conservation research. Economics can help identify least, or low, cost opportunities for conserving species as is demonstrated in this article, even if it is not always possible to demonstrate their economic value convincingly.