The role of the chloride ion in iron corrosion is assessed from the viewpoint of developing the concept that would lead to a general explanation. The experimental observations relating to such factors as the oxygen effect, concentration, cation, temperature, and pH dependence are reviewed. Some of the observations on the relative corrosion effects of various ions are described. These experimental factors are discussed in terms of oxide film theory (including colloidal films) and adsorption mechanisms. It is concluded that the anion effects can best be interpreted in terms of the complexes that are formed between Fe and the anion. In neutral chloride solutions, the complex is transitory unless it is “trapped” in a pit or crevice. In many acid solutions, halide complexes (compounds) are stable enough to inhibit corrosion.