Plasticity of crystalline solids (or materials) is due to the action of various microscopic mechanisms of plastic deformation, such as emergence, motion and extinction of dislocations. The relative importance of each of these mechanisms is determined by external conditions: temperature, load, speed of deformation. Defects in the crystal structure (vacancies, interstitials and impurity atoms), which may impede or facilitate the motion of dislocations, have a significant effect on plasticity. For polycrystalline solids the grain boundaries are also an important contribution to plasticity. Grain boundaries prevent the spread of dislocations and, as a rule, harden crystalline bodies at low temperatures. To the contrary, at high temperatures, the presence of the boundaries, which are sources or sinks for defects, increases plasticity.

Plasticity of simple amorphous solids is connected with the diffusional rearrangements of atoms and molecules and is similar to the flow of liquids.

In the case of material extension along a single axis, the measure of plasticity is the relative elongation (deformation) at rupture. In the case of a complex stress state of a material, the mechanics of solids employs the mathematical apparatus of tensor analysis for the evaluation and calculation of plasticity.

Various mechanical theories of plasticity are used to calculate the structural elements: the theory of rigid plastic body (perfect plasticity), the theory of elastic plastic body, the theory of strain-hardening elastic plastic body, etc. Currently, a theory of singular plasticity and various theories that take into account strain dynamics and temporal scales are being actively developed.