Crystallographic slip requires a very high stress if the crystal planes are displaced rigidly. If the displacement proceeds sequentially, however, by motion of a crystal dislocation only low stresses are needed for the shape change. The macroscopic deformation parameters, flow stress and strain rate can be directly related to the dislocation properties, force on a dislocation (Peach-Koehler equation) and dislocation velocity (Orowan equation).