The Earth’s internal heat is its greatest source of energy. It powers global geological processes such as plate tectonics and the generation of the geomagnetic field. ‘The Earth’s heat’ explains that the internal heat arises from two sources: the decay of radioactive isotopes in crustal rocks and the mantle, and primordial heat left over from the planet’s fiery formation. The internal heat has to find its way out of the Earth. The three basic forms of heat transfer are radiation, conduction, and convection. Heat is also transferred in compositional and phase transitions. Heat transport by conduction is most important in solid regions of the Earth, while thermal convection occurs in the viscoelastic mantle and molten outer core.