This volume of the Encyclopedia of Sustainability Science and Technology, Second Edition, describes technologies that actively convert solar radiation into useful heat in a temperature range from just above ambient up to more than 1,000°C. Applications cover a broad range of energy services such as space heating, cooking, domestic hot water supply, electrical power generation, and high temperature thermochemical processes. The major developments that have led to currently available technologies for solar thermal energy applications were initiated mainly after the first oil shock in 1973. Solar thermal energy is widely used already for heating purposes (water, space) in the low temperature range up to about 100°C employing mainly nonconcentrating collectors, whereas higher temperatures can be achieved with more sophisticated solar collector technologies. Temperatures over 200°C typically require concentrating solar radiation using mirror systems. Several different technologies are described in detail in this volume, including solar collector systems in the lower temperature range, the direct use of solar radiation for food processing, namely cooking and drying, the production of electricity through conversion of solar radiation first to heat, driving a mechanical conversion system coupled to an electric generator, the use of solar radiation to drive chemical processes, and many more.