Hardening and depth of hardening of steel parts are critically important material and process design parameters. Hardenability is the composition-dependent property of a steel that describes its ability to harden by martensite formation and is related to parameters such as austenitizing temperature, cooling rates after austenitizing, and part size and geometry. A selective overview of the experimental and predictive procedures to determine hardenability is presented. This discussion covers the breadth of steel hardenabilities, including those of shallow-hardening, low-carbon steels, plain-carbon and low-alloy medium-carbon steels, air-hardening tool steels, and carburizing steels. Sufficient detail and references are provided to enable the reader to adequately understand and practice the methods discussed.