The mountainous regions of mainland South-East Asia have been a classic ground both for the study of shifting agriculture systems, 1 and for their condemnation. National policy in Laos, 2 Vietnam, 3 Thailand 4 and China 5 has all been based on the belief that traditional practices of shifting agriculture are wasteful and destructive. Development policy is carried out on the assumption that certain land use practices, such as planting rubber in South-Western China and fruit trees in northern Thailand, building terraces and planting contour vegetative strips, or conversion of flat land into paddles for wetland rice are the basis for sustainable land use. Under certain conditions, however, these and similar solutions may actually lead to more problems than they solve. In this paper we use the agroecosystem as a framework of analysis, and suggest that changes affecting elements within an agroecosystem can render the entire system unsustainable. We pay attention particularly to the growth of the human population, to increasing commercialization, to efforts made to find substitutes for opium as a cash crop, and to the rapidly emerging consequences of transborder trade in the region. Our report is based on recent studies in Thailand and Laos. 6