The Earth is about 4.6 billion years old. In terms of its thermal regime, the planet is in the process of cooling. However, to have reached its current state, the Earth and the other objects making up the Solar System went through a number of stages such as the accretion of the planet from dust of the solar nebula, the formation of the magma-ocean, stratification of matter by density, solidification of the magma-ocean, formation of the lithosphere which is taking place today, periods of increased volcanic and metamorphic activity, numerous tectonic processes with global and regional significance (obduction, subduction, orogeny, etc.), heat production by short-lived and long-lived radioisotopes, and numerous other features and processes related to thermodynamic and temperature conditions. In this Chapter are analyzed such fundamental phenomena as sources of the thermal energy in the Earth's interior, geothermal gradient, density of heat flow, heat flow and geological age, mantle heat flow, temperature distribution inside the Earth and other Earth-thermal interactions.