Induction and flame hardening are methods of hardening the surfaces of components, usually in selected areas, by the short-time application of high-intensity heating followed by quenching. These processes are used when gear teeth require high hardness, but size or configuration does not lend itself to carburizing and quenching the entire part. This chapter focuses on the processes involved in the induction and flame hardening, covering the applicable materials, hardening patterns, preheat treatment, quenching, tempering, surface hardness, case depth, hardening problems, dual-frequency process, and applications.