Gas (atmosphere) carburizing is the de facto standard by which all other surface hardening techniques are measured and is the emphasis of this chapter. Initially, the chapter describes the process and equipment for gas carburizing. This is followed by sections discussing the processes involved in quenching, hardening, tempering, recarburizing, and cold treatment of carburized and quenched gears. Next, the chapter reviews the selection process of materials for carburized gears and provides information on carbon content, properties, and core hardness of gear teeth. The problems associated with carburizing are then covered, followed by the processes involved in heat treat distortion...