This chapter covers the fundamentals of heat treating. It begins with a review of the composition, classification, and properties of iron and steel, the phases of the iron-carbon system, and the basic types of heat treatments. It then discusses the topics of hardness and hardenability, the role of carbon in the hardening of steels, the process of austenitization, and the influence of cooling rate on subsequent transformations. The chapter also explains how induction heating affects residual stress, distortion, and grain size.