This chapter will focus on quenching technology for steel heat-treating applications. Quenching involves the heating of steel to an elevated temperature and is followed by a controlled cooling process to achieve the desired microstructure and properties. This process depends on the type of quenchant and bath conditions used. The function of a quenchant is to improve heat transfer extraction from the hot metal during the cooling process and to produce the desired metallurgical transformation. The selection of quenching media must consider the quenching severity required to optimize the desired transformation, while at the same time providing optimal residual stresses and distortion control. The most common quenchants include mineral oil, water, aqueous polymer solutions, nitrogen, helium, and others. Aspects of steel quenching technology, such as fundamentals of processes, hardenability, common quench media, cooling curve analysis, and bath maintenance will be discussed in this chapter.