After a brief review of the importance of diffusion in most metallurgical processes, it is shown that the determination of parameters with a precise physical meaning requires the setting up of experiments of a very special type. This results in a certain discrepancy between the practical value of the measurements and the possibility of their theorectical interpretation. Diffusion results from the random migration of atoms in the crystalline lattice. The analysis of this movement makes it possible to write the diffusion coefficient as a function of the atomic distances and of certain jump frequencies, and in a second stage to find a variation with the temperature according to an Arrhenius law. The factors which enter in to the expression of this law depend on the specific mechanism of diffusion (interstitial, vacancy, etc.) In most materials, diffusion does not result soley and simply from this process of intracrystalline migration. The importance of the short circuits constituted by dislocations, grain boundaries and all the interfaces, and external surfaces cannot be ignored.