The 'facts' about environmental problems are usually left to the scientists to discern. There is increasing recognition, however, that scientific reality is not generated from objective study but rather, it is socially constructed. Such a view gives greater credibility to alternative, competing perspectives. This paper presents various discourses on land degradation in the Uluguru Mountains, Tanzania. It considers the foundations of, and influences upon, perceptions and assessments of degradation made by colonial administrators, scientists and local people. It also examines the way power is used to legitimise different knowledge claims.