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 (∇T > 60 °C / km) produce electricity in the 6-7c/ kWherange. For low-grade systems (∇T < 40 ° C / km) to become commercially attractive, much higher reservoir productivity levels and / or substantially lower drilling costs are required.