Derivations of three types of growth laws which express rates of formation of films on metal surfaces are given. The linear law, , is obtained in cases where the film is porous and inward leakage of oxygen or other non‐metals can occur. The parabolic law, , is obtained when the film is non‐porous and possesses ionic and electronic conductivity. The growth rate, in this case, is governed by outward ionic migration under a potential gradient and is usually associated with vacant sites in the cationic lattice. The logarithmic law, , occurs when the conditions needed for parabolic thickening are absent. It is believed to be caused by the outward passage of interstitial matter through flaw‐paths of loose structure in the outer part of the oxide film. The actual rate of passage along a given path is regulated by the rate of crossing a less pervious inner film of constant thickness and is independent of the thickness of the outer film, provided that no obstruction occurs in the path through the outer film.