This article presents the traditional knowledge of an ethnic group of sea nomads generally known in Thailand as Chao Lay. The Moken once led a nomadic marine life. They have developed their traditional knowledge and belief system over several centuries. This practical knowledge has been obtained through interaction with local ecosystems and from observation and experimentation in everyday life. The Moken have intimate knowledge relating to the sea and the forest, and they have elaborated boat-building skills and other technologies that allow them to make their living from the sea, coastal areas, and islands. This traditional knowledge and attendant practices represent a form of natural resources management and conservation. It comprises: 1) knowledge and skills that depend upon simple technologies that have minimal impact on the natural environment and its resources; 2) a nomadic life with frequent displacements that allow the Moken to rotate their foraging grounds and prevent overuse and degradation of specific areas; 3) knowledge about numerous forest and marine species – their characteristics, behaviour, habitats and eco-niches – which enables the Moken to make use of a diversity of local ecosystems; 4) a hunter-gatherer livelihood focusing primarily upon subsistence, with little accumulation of material goods, and finally 5) a philosophy and belief system that holds that natural resources are not individually owned, but rather are to be shared by everyone without restrictions on access. The sharing ethic is very strong in the Moken community and resources are shared not only with fellow humans but with supernatural beings as well. This system of traditional knowledge, know-how and representations has never been recognised nor respected. Rather than a form of natural resources management and conservation, it has been misinterpreted as part and parcel of a “primitive”, underdeveloped, and materially poor livelihood. For mainstream society, development for the Moken necessarily requires the termination of their “primitive” life and the embracing of modernity. Through this pathway to development, however, the Moken are likely to lose their traditional knowledge and the sustainable livelihood that has ensured their physical and cultural well-being for centuries.