The indentation size effect is one of several size effects on strength for which “smaller is stronger.” Through use of geometrically self-similar indenters such as cones and pyramids, the size effect is manifested as an increase in hardness with decreasing depth of penetration and becomes important at depths of less than approximately 1 μm. For spherical indenters, the diameter of the sphere is the most important length scale; spheres with diameters of less than approximately 100 μm produce measurably higher hardnesses. We critically review experimental observations of the size effect, focusing on the behavior of crystalline metals, and examine prevailing ideas on the mechanisms responsible for the effect in light of recent experimental observations and computer simulations.