Mobile livestock herders have long been seen as the main culprits of overstocking and rangeland degradation. In recent years, however, anthropologists and ecologists have argued that African pastoralists have developed sustainable modes of pasture management based on a sound knowledge of savanna ecosystems. Comparing indigenous knowledge on species grazing values, plant succession, and ideas about the causes for environmental change in two African pastoral societies (the Kenyan Pokot and the Namibian Himba), it is shown that their knowledge is indeed fine-grained and complex but at the same time socially constructed and embedded in ideology. Keywords: African pastoralists, indigenous knowledge, environmental degradation.