It is difficult to explain, on the basis of the usual theory of the structure of solid solutions, the effect of foreign atoms on the solid solution matrix when~ ~or example, one atom of the dissolved substance is surrounded by hundreds or even thousands of solvent atoms. The reason for this effect becomes clearer when some refinements are introduced into the theory of the structure of solid solutions. It is known that there are considerable lattice distortions at grain boundaries, which are therefore sites of higher than average free energy. The special physical state of layers of atoms at the surfaces of grains is due to their asymmetrical position relative to the surrounding atoms. Gibbs was the first to establish a thermodynamic relation between surface tension and the change in concentration of the solution close to the surface. Qualitative and quantitative checks on the Gibbs equation, applied to liquid solutions, have given completely satisfactory results [i]. Gibbs ideas have been further developed by F~onobeevskii and others~who have shown that local changes in free energies of systems caused by tensile or compressive stresses lead to a redistribution of atoms within the solid solution. By developing and generalizing these ideas we conclude that the presence of any defects or lattice imperfections inside a real crystal (grain and block boundaries, inclusions, vacancies, distortions etc.) create the conditions for a redistribution of dissolved elements. This leads to the formation of heterogeneous structures, and under certain conditions, to heterogeneous equilibria: it should be emphasized that diffusion processes, leading to the formation of heterogeneous structures, proceed according to the laws of thermodynamics. From this viewpoint, the fact that under definite conditions a homogeneous solid solution, in the stable state, may become heterogeneous, should cause no surprise. The composition and state of the boundary layers exert a substantial effect on the properties of alloys and on their behavior at low and high temperatures. It has been found that in many cases atoms at grain boundaries are many times as mobile as those inside grains. This is of enormous importance, since diffusional processes at grain boundaries are responsible for aging, temper brittleness, blue brittleness and probably affect the mechanisms of phase transformations and recrystallization.