Publisher Summary The hardness of the surface of components is an important property affecting their tribological performance. For components with nonconformal contacts—such as rolling bearings and gears—the hardness, and the corresponding compressive strength, of the surface material must be above a critical value. This chapter focuses on the measurement of hardness of the surface components. The hardness of component surfaces is measured by indenting the surface with a small indenter made from a harder material. The hardness can then be inferred from the width or area of the indentation or from its depth. The chapter tabulates an approximate comparison of the scales of hardness.