This lecture covers the concept of heterodiffusion in metals, focusing on examples of metals with solutes at infinite dilution and their correlation with the valence of the solvent-solute. It also discusses interstitial solutes such as C, N, O and the effect of temperature on the activation energy of direct interstitial diffusion. The presentation delves into diffusion mechanisms in solids, including self-diffusion in metals, diffusion in compound alloys, and the diffusion of electrolytes in liquids.