THIS CHAPTER WILL FOCUS ON QUENCHING TECHNOLOGY FOR STEEL HEAT TREATING APPLICATIONS. Quenching is the process of cooling metal parts to achieve the desired microstructure, hardness, strength or toughness. Quenching can produce both desirable and undesirable residual stresses and distortion in addition to cracking. Steel, for example, is heated to the austenitizing temperature, that temperature where the austenite microstructure is formed. To obtain optimum hardness, strength, and toughness, the maximum amount of martensite transformation microstructure is desired. The primary function of the quenching medium is to control the rate of heat transfer from the surface to optimize the microstructure while minimizing undesirable features such as cracking and distortion [1].