This article discusses the classification of carbon steels based on carbon content, and tabulates the compositional limits of medium- and high-carbon steels based on the AISI code and other similar codes. It describes recrystallization annealing and spheroidizing of carbon steels, and discusses the classification of carbon steels for heat treatment. The article also discusses the estimation of continuous cooling curves from isothermal transformation curves. It provides information on the Jominy end-quench test and the Grossmann method and the procedures to increase hardenabilty of carbon steels. The article includes information on the purpose of tempering and heat treating guidelines for different grades of steels, including cast carbon steels.