Carbonitriding is a modified form of gas carburizing. It is performed in a closed furnace chamber with an atmosphere enriched with a gaseous compound of carbon and nitrogen. This chapter provides information on the carbonitriding of steels, the applications of carbonitriding, the typical case depths, and the hardenability of carbonitrided parts. In addition, the chapter also discusses the processes involved in quenching, tempering, and distortion of carbonitrided steels.