This article provides useful information on the selection of steels for heat treatment in order to achieve the required hardness. It discusses the effects of alloying elements on hardenability using the Grossmann's concept, and presents a discussion on the effects of alloying elements in hot-worked and cold-drawn steels. The article focuses on the selection of carbon and alloy steels based on the function of the alloying elements, and discusses the specific effects of alloying elements in steel in a tabulated form. The depth and degree of hardening (percentage of martensite) are dictated by the engineering stress analysis. Mechanical properties of quenched and tempered steels develop similar tensile properties for all practical purposes for all compositions with the same hardness. The article also provides information on the selection of steels to meet the required hardness, and elucidates the concept of hardenability for wear resistance with the help of graphs.