The oxidation of metals is, by definition, a reaction between a gas and a solid which usually produces a solid reaction product. At first glance, this would therefore seem to be a very simple process but, in fact, it is considerably more complex. One would like to think that the reaction product, i.e., the scale that forms on the metal, acts as a physical barrier between the reactants, and that the reaction should thus cease once the barrier is established. We know that this is unfortunately not the case, because transport of matter through the scale allows the reaction to continue. We also know that, because of density-differences between the metal and its oxide, the scale may not be sufficiently complete in coverage or may not adhere to the substrate because of cracking, spalling and detachment (wrinkling). In some extreme cases, the scale may even be a liquid which simply drips from the surface, or it may volatilize at operational temperatures. The reaction between a gas and a metal is truly very complicated.