A conversion coating is an inorganic or organic–inorganic hybrid surface film conveying specific desirable functions to a material, formed by a spontaneous reaction between the material and an electrolyte. This chapter reviews the fundamental physical chemistry of the conversion coating process and overviews the variety of different conversion coatings. Emphasis is placed on the conventional phosphating and chromating process while reviewing alternatives such as trivalent chromium, titanate, zirconate, and vandate, rare earths, layered double hydroxides, and hybrid inorganic-organic species. The chemistry and formation mechanisms of each conversion coating are discussed in this chapter. Analytical techniques to understand the formation mechanism of the conversion coatings are also summarized.