Early theories of passivity are summarized, and two definitions of passivity are noted. The theories can be classified as: a) metal modification, b) reaction rate, c) oxide film, and d) adsorption. More recently, studies of alloys have shown that the properties of the passive film are dependent on the composition of the alloy, though the film largely includes oxides. Present-day research involves studies of stoichiometry, film composition and chemical and physical properties. -- AATA