Physical definition of energy, heat, temperature and its measurement, principal phenomena of thermodynamics, heat generation, specific heat capacity, heating value, thermal expansion, heat transfer, heat flux density. Source of heat on the Earth, the role of heat in the energy transfer and particle movement of outer and inner geospheres, in forming of minerals, rock, raw materials, Relation between the magmatism, tectonism and geothermal energy. Temperature and heat flux density field of the Earth. Geothermal methods in geological and geophysical research. Hyperthermal areas, areas with high enthalpy and low enthalpy, geothermal systems, thermal karst areas, thermal water fields, geysers, thermal springs, etc. Possibilities of the heat production from deep reservoirs, hot dry rocks and springs. Thermal wells, borehole heat exchangers, heat pumps, geothermal power plants, heating and warming sanitary hot water, heat losses, communal and agricultural utilization in domestic and international examples.