Geothermal energy is a renewable resource that enables a sustainable and near-zero-CO2- emission energy economy. Geothermal energy comes in various forms and potential uses, from kilometers-deep hot geothermal reservoirs delivering steam for electricity to meters-deep geothermal installations that supply direct heat for individual houses. This course explores the range of geothermal resources and their potential use, illustrating also applications in Saudi Arabia. The lectures cover both the fundamental physics of geothermal heat generation and transport in the Earth, the needed geological conditions for the existence of geothermal reservoirs, and the corresponding prospecting for and quantification of such reservoirs. The course also introduces basic engineering systems for extracting and using geothermal energy systems as part of a sustainable energy mix for small-scale (individual house) to large-scale (communities and cities) developments. Furthermore, fit-for-purpose drilling techniques will be discussed.