Work hardening is an increase in mechanical strength due to plastic deformation. In metallic solids, permanent change of shape is usually carried out on a microscopic scale by defects called dislocations which are created by stress and rearrange the material by moving through it. At low temperature, these defects do not anneal out of the material, but build up as the material is worked, interfering with one another's motion and thereby increasing strength. See the main article on cold work.

Any material with a reasonably high melting point can be strengthened in this fashion. It is often exploited to harden alloys that are not amenable to heat treatment, including low-carbon steel. Conversely, since the low melting point of Indium makes it immune to work hardening at room temperature, it can be used as a gasket material in high-vacuum systems.

Often, cold work is carried out by the same process that shapes the metal into its final form, including cold rolling (contrast hot rolling) and cold drawing. Techniques have also been designed to maintain the general shape of the workpiece during work hardening, including shot peening and constant channel angular pressing ). A material's work hardenability can be predicted by analyzing a stress-strain curve, or studied in context by performing a hardness test before and after the proposed cold work process.