The reaction of a metal or alloy with an oxidising environment to form a scale often involves a diffusion process as the rate limiting step. The most protective oxide scales are slow growing, adherent to the substrate, and free of cracks or pores. The growth of these scales is typically by solid state diffusion of metal or oxygen ions that move via point defects in the oxide lattice. In 1933, C. Wagner established a scientific basis for oxidation processes controlled by solid state diffusion, with his celebrated derivation of the parabolic rate constant, which connects scaling rates, diffusion coefficients, point defect concentrations, point defect types, and effect of external parameters, e.g. pO2. These aspects are discussed in this paper. The importance of the Wagnerian theory is to provide a relatively simple model upon which more comprehensive models may be built. For many applications, the rate of degradation of the metal or alloy, owing to oxidation by lattice diffusion would be quite acceptable. Several examples of oxidation processes controlled by vacancy and/or interstitial diffusion will be discussed.