The use of geothermal resources can be subdivided into three general categories: high-temperature (>300°F [>150°C]) electric power production, intermediate- and low-temperature (<300°F [<150°C]) direct-use applications, and ground-source heat pump applications (generally<90°F [<32°C]). This chapter covers only direct use (including wells, equipment, and applications) and ground-source heat pumps. Design aspects of the building heat pump loop may be found in Chapter 9 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment. GROUND-SOURCE HEAT PUMPS Terminology General Information Ground-Coupled Heat Pump Systems Using Water-Based Heat Transfer Fluids Ground-Coupled Heat Pump Systems Using Refrigerant-Based Heat Transfer Fluids (Direct Exchange) Open-Loop Groundwater Heat Pump System Components Open-Loop Groundwater Heat Pump System Design Surface Water Heat Pumps DIRECT-USE GEOTHERMAL ENERGY Resources Fluids Present Use Design Cost Factors Materials and Equipment Residential and Commercial Building Applications Industrial Applications Renewability