Energy options for the medium- to long-term future have to be studied in terms of demand, resources, technologies, strategies, and constraints. A joint effort of the International Atomic Energy Agency (IAEA) and the International Institute for Applied Systems Analysis (IIASA) is directed toward assessing risks inherent in the deployment of any energy system. A combination of three energy supply technologies is available to satisfy projected requirements. These are nuclear, solar, and (to a limited extent) fossil fuel energy conversion systems.