The through-hardening process is generally used for gears that do not require high surface hardness. Four different methods of heat treatment are primarily used for through-hardened gears. In ascending order of achievable hardness, these methods are annealing, normalizing and annealing, normalizing and tempering, and quenching and tempering. This chapter discusses the processes involved in the through-hardening of gears. It provides information on designing procedures, hardness, distortion, and applications of the through-hardened gears. The chapter presents a case history on the design and manufacture of a through-hardened gear rack.