Atomic diffusion in metals was first discovered some sixty-five years ago, and since then a considerable wealth of data has ac­ cumulated on diffusion in various systems. However, work prior to about the year 1940 is now mainly of historical interest, since ex­ periments were often carried out under experimental conditions and with methods of analysis leading to uncertainties in inter­ preting the measured diffusion coefficients. Data on diffusion rates are of importance in processes which are controlled by rates of atomic migration such as growth of phases and homogenization of alloys. In addition diffusion plays an important part in theories of such phenomena as oxidation, plastic deformation, sintering, and creep. A tremendous advance in diffusion studies was made possible by the availability of radioactive isotopes of sufficiently high spe­ cific activity after the second world war. Measurements of self­ diffusion rates then became possible using radioactive isotopes having the same chemical properties as the solvent material, and it also became possible to study tracer impurity diffusion when the concentration of the impurity is so small as not to alter the chemi­ cal homogeneity of the system. In the last ten to fifteen years the purity of materials used in diffusion studies has increased con­ siderably and the methods of analysis have become more stand­ ardized.