Plastic deformation of crystals is carried by lattice defects: the migration of vacancies is responsible for deformation by transport of matter, the propagation of dislocations is responsible for deformation by slip, in polycrystals grain-boundary motion may also be a source of deformation.

Vacancies are point defects: vacant sites in the crystal lattice. For entropic reasons there exists an equilibrium concentration of vacancies dependent on the temperature. The vacancies migrate by exchange with neighbouring ions; their migration obeys diffusion equations (Fick's laws) identical to those ruling the diffusion of heat.

Dislocations are linear defects at the boundary between an area on which slip has taken place and the rest of the crystal, as yet ‘unslipped’. They create an internal strain field and stress field extending through the whole crystal and whose strength decreases as the inverse of the distance to the dislocation. It is through their strain field that dislocations ‘see’ an applied stress and move, thus increasing the slipped area. Dislocation motion is impeded by a thermally activated lattice friction force (Peierls' force) and by obstacles. Orowan's equation is a microscopic equation of state relating the strain-rate to the dislocation density velocity.

Grain boundaries in a chemically homogeneous material are two-dimensional defects separating grains whose lattices have different orientations. They can often be described as arrays of dislocations. Recrystallization is a change in the granular structure involving motion of grain boundaries.