The objective of this paper is to suggest a methodology that will help us to determine if the present rate and direction of technological change is compatible with the development of a sustainable society. We combine two perspectives on technology assessment. The first focuses on current techno-economic trends and the second on long-term resource and environmental constraints to the diffusion of a new technology. We apply our approach to the case of solar cells. Based on an analysis of technology, actor and market dynamics we suggest that thin-film solar cells are about to dominate the industry. Within the thin-film family, there is competition between alternative designs. The diffusion of three of these will, however, be limited by resource, and perhaps emission, constraints. One design (a-Si) fares much better in terms of these constraints but is less efficient. Three policy issues are identified. First, the diffusion of solar cells is not yet self-sustained and further policy intervention is required. Second, the problems of the current thin-him designs suggest that there is a need for policies both to sustain variety and to balance that requirement with the short-term requirement of cost reduction. Third, policy must ensure that a diffusion of solar cells containing scarce metals does not lead to an erosion of environmental constraints.