Mechanisms of ultrafine structure formation during severe plastic deformation are considered in the work. It has been shown that during the deformation, the structure formation is determined by the mechanisms that lead to an increase in defect density and the relaxation mechanisms that lead to a decrease in the density of defects. The concept of an intensified movement of defects is involved in the work to describe the experimental data on changes in the structure and properties of materials at large plastic deformations, that provides a fairly complete picture of the formation of structure of metals and alloys.