This chapter describes geothermal energy as a renewable energy source, its use in producing heat, cooling, and electricity, and the main applications and technologies, including underground thermal storage and co-production of energy and mineral resources, e.g., lithium. Geothermal energy is the thermal energy stored underground, including any contained fluid, that is available for extraction and conversion into energy products. Electricity generation usually requires geothermal fluids at temperatures of over 100 ℃. For heating and cooling purposes, geothermal systems that span a more comprehensive range of temperatures are used in various applications such as space and district heating and cooling, spas and swimming pools, and heating for greenhouses and soil, aquaculture ponds, industrial processes, and snow melting. Geothermal energy is immune from weather and seasonal variations and can be produced continuously (base-load) or adapted to energy demand, providing a flexible and “smart” renewable energy source. To guarantee the sustainable use of geothermal energy, the consumption rate should not exceed the generation rate so that the heat removed from the underground can be replaced within a similar time scale. For this reason, geothermal plants typically produce energy below a certain level. Moreover, potential impacts on the environment are controlled, mitigated, and managed by diverse technologies. Geothermal energy is vital in meeting international climate targets and provides socio-economic benefits to communities and industries. Such advantages are not widely known, and geothermal projects in some regions are confronted with a negative perception. There is a need for a strengthened policy framework to improve the social dimension of projects and a paradigm change for the developers, putting the public in a central position.