The Solar Analyst is a new ArcView GIS extension that provides convenient access to a powerful array of solar simulation tools. Maps of incoming solar radiation (insolation) are calculated using digital elevation models (DEMs) for input. A fast view-shed algorithm accounts for the influences of surface orientation (slope and aspect), elevation, shadows, and atmospheric conditions on direct and diffuse insolation. Insolation maps have applications in such diverse disciplines as agriculture, forestry, hydrology, ecology, and meteorology. For example, in ecological studies at the Rocky Mountain Biological Laboratory, Colorado, insolation maps have proven valuable for understanding vegetation distribution and potential effects of global warming.