Even as children we understand that things such as cake, cotton, and sponge are soft, while stones, metals, and bricks are hard. The most basic method for detecting hardness or softness is by the sense of touch. As materials scientists, however, we need to quantify the amount of hardness in order to have a systematic method of deciding where a hard material can be used, and where a soft one is more applicable. While the sense of touch does enable us to distinguish the soft from the hard, how do we use the mathematical language to distinguish them? This chapter will provide mathematical descriptions of hardness and describe various test equipment developed for measuring hardness. We will first focus on the commonly used macroscopic hardness testing methods, macrostatic indentation, macrodynamic indentation, and macroscratch tests for measuring macroscopic static indentation hardness, dynamic indentation hardness, and macroscopic scratch indentation such as Brinell hardness, Rockwell hardness, and Vickers hardness. At the same time, we will introduce methods for measuring microhardness and nanohardness in order to satisfy our need to know the hardness of microsized and nano-sized materials. Lastly, we will briefly discuss an interesting hardness phenomenon that occurs with nano-sized materials called the "size effect," from which we can gain valuable insight into the fundamental physics behind the macroscopic and microscopic mechanical properties of materials.