This article commences with a brief introduction on the hardenability of carburized steels, and then reviews the factors used in the selection of carburizing steels and heat treatment methods. The factors include quench medium, stress considerations, case depth, and type of case. The article provides information on steels for carburized gears with emphasis on gear design requirements, selection process, selection of carbon content, case and core hardness, microstructure, and toughness and short-cycle fatigue.