This article gives a tutorial survey of the xerographic process and the relevant physical phenomena involved in the formation of the latent image. Emphasis is on experimental results on amorphous selenium, but other materials will be described in order to explain properties which are either general to the disordered state or characteristically different from those of wide-band semiconductors. The discussion includes a description of the Onsager mechanism of charge generation, transient dispersive transport and recent models of defects in amorphous semiconductors. The article ends with a description of recent results related to the control of defects in chalcogenide glasses.