The present status of the theory of the late stage of diffusive decomposition of multicomponent solid solutions, owing to diffusive interaction of macrodefects, is reviewed. Methods for solving the systems of equations describing the decomposition process are analyzed. The evolution of the critical dimensions of precipitates and the transformation of an arbitrary distribution function into a universal function for different mechanisms of mass transfer are studied in detail. The conditions for coexistence of phases in the presence of decomposition of multicomponent systems are examined. The results of the theory are compared with experiment.