Our understanding of the plastic behavior of solids has lagged well behind the knowledge gained in other branches of solid state physics. Electrical, thermal, elastic, magnetic, and optical properties are explained in terms of a few basic concepts such as the vibrations of atoms and the motion and spin of electrons. We understand fairly well what happens to the atoms and electrons when, for example, a metal absorbs heat or light or carries current in an electric or magnetic field. It is considerably more difficult, however, to discover what basic atomic mechanisms operate when a metal fractures or deforms plastically. Nevertheless the problems of strength and plasticity are highly important technologically and become more so with the increasing importance of jet engines, gas turbines, and other devices that require materials to withstand large forces at high temperatures.