This chapter discusses the general principles of measuring hardness and hardenability of steel. The discussion begins by defining hardness and exploring the history of hardness testing. This is followed by a discussion on the principles, applications, advantages, and disadvantages of commonly used hardness testing systems: the Brinell, Rockwell, Vickers, Scleroscope, and various microhardness testers that employ Vickers or Knoop indenters. The effect of carbon content on annealed steels and hardened steels is then discussed. A brief discussion on the concept of the ideal critical diameter and austenitic grain size of steels is also provided to understand how one can calculate and quantify hardenability. The processes involved in various methods for evaluating hardenability are reviewed, discussing the effect of alloying elements on hardenability.