The monograph discusses the processes accompanying mutual diffusion in two-component systems. Considerable attention is paid to the processes and effects that result from the relaxation of various signs of non-equilibrium arising in the diffusion zone due to the difference in the kinetic and thermodynamic characteristics of the components of the diffusion pair. The flow of matter in the diffusion zone (Kirkendall effect), the origin and growth of pores, the formation of dislocation structures, dimensional and surface effects are considered. All considered phenomena significantly determine the real structure of the diffusion zone.