The extraction of heat or-thermal energy from the Earth-heat mining-has the potential to play a major role as an energy supply technology for the 21st century. This paper looks specifically at the potential for hot dry rock (HDR) geothermal energy in the United States. A generalized multiparameter economic model was developed for optimizing the design and performance of HDR geothermal systems. Assuming reservoir productivity comparable to existing hydrothermal systems, high-grade HDR systems (del T > 60 degrees C /km) produce electricity in the 6-7 cent/kWh(e) range. For low-grade systems (del T < 40 degrees C / km) to became commercially attractive, much higher reservoir productivity levels and / or substantially lower drilling costs are required.