Heat treatment is part of the steel processing. The reason to use heat treatment is explained and the different heat treatment are introduced. The quenching in different media is explained and the stages of tempering is introduced. Then, the surface hardening treatment is introduced. In this topic the Carburising, Nitriding and Induction hardening is detailed. Finally, the different types of furnaces and control of product obtained into heat treatment is described and the applications of heat treatment into different kind of steels is described.