Models and mechanisms which have been proposed for silver sulfide and silver-gold sulfide sensitization are reviewed. These sulfides are not involved in the primary photochemical processes of the direct photolysis theories of Sheppard, Gurney and Mott, Hamilton, Kanzaki and Tadakuma, and Tani. In these theories, photoelectrons and positive holes are created in pairs and the role of sulfide sensitization in trapping photoelectrons is emphasized. With a chemically sensitized silver halide emulsion grain, there are, however, a large number of equivalent sites at which electron trapping can occur and the theories provide no process for the efficient formation of a development center by the separation of a small number of silver and gold atoms at one particular site. According to the photoaggregation theory, chemical sensitization results in the formation of small monolayer islands of Ag₂S and (Ag,Au)S molecules at surface sites with Ag₂ molecules adsorbed around their edges. These sensitizing molecules are dissociated by the annihilation of energy quanta with creation of photoelectrons and release of Ag_o⁺ or Au_o⁺ interstitial ions. A development center is formed at an Ag₂ latent image growth nucleus precursor by two effective events. In the first event, a photoelectron and interstitial ion combine with an Ag₂ molecule to form a Ag₃ or AuAg₂ molecule that adsorbs a Ag⁺ ion to give an Ag₄⁺ or AuAg₃⁺ latent image growth nucleus. In the second event, this positively charged nucleus attracts and traps a photoelectron and the positive charge is restored by the effective adsorption of an Ag_o⁺ ion giving an Ag₅⁺ or AuAg₄⁺ development center of minimum size, adsorbed to a sulfide monolayer island. This center grows by the repetition of these electronic and ionic events and this provides an efficient concentration process.