Diffusion on nanoscale is still not well understood even if the role of structural defects (dislocations, grain boundaries) can be neglected. In this case “only” such principal difficulties arise as the problem of the transition between the continuum and discrete description, or the problem of the non-linearity (due to the strong concentration dependence of the diffusion coefficients). Furthermore the segregation kinetics are also always related two the redistribution of atoms on nanoscale and thus for the understanding of them the above questions should also be clarified. In addition, the size effects influence the equilibrium isotherms as well.