This paper reviews the various theories of the motion of light interstitials in metals. It discusses the formulation of quantum theories and their relation to each other and to the classical theories. The physical assumptions are summarised, and a comparison of theory and experiment is made.The theory successfully predicts (1) the distinction between bcc and fcc host lattices, (2) the relation between activation energy and volume of solution in bcc hosts, (3) the absolute magnitudes of the activation energies, (4) the temperature dependence of the diffusion rate, (5) the existence of isotope effects in electro‐ and thermomigration, and (6) the Zener relations. Topics which are not yet understood are discussed.