Nanoindentation has become a commonplace tool for the measurement of mechanical properties at small scales, but may have even greater importance as a technique for experimental studies of fundamental materials physics. With high-resolution load-displacement data, discrete events including dislocation source activation, shear instability initiation, and phase transformations can be detected during a nanoindentation test. Recently-developed capabilities in, for example, high-temperature nanoindentation testing and in situ imaging of the indented volume, offer new quantitative details about these phenomena, and present many opportunities for future scientific inquiry.