The word ‘Geothermal’ comes from the combination of the Greek words ge, meaning earth, and therm, meaning heat. This heat manifests itself on the surface in the form of volcanoes, geysers and hot springs. Geothermal resources are concentrations of the earth’s heat, or geothermal energy, that can be extracted and used economically now or in the reasonable future. The earth contains an immense amount of heat, but the heat generally is too diffuse or too deep for economic use. Hence, the search for geothermal resources focuses on those areas of the earth’s crust where geologic processes have raised temperatures near the surface such that the heat contained can be utilised. Currently, only concentrations of heat associated with water in permeable rocks can be exploited economically. These systems are known as hydrothermal–geothermal systems. This chapter will discuss where the earth’s thermal energy is sufficiently concentrated for economic use, the various types of geothermal systems, the production and utilisation of the resource and the environmental benefits and challenges of geothermal production.