In xerography, a corona charged photoconductor film transforms an optical image into an electrostatic image that is subsequently developed. A variety of photoconductor materials has been employed in commercial reproduction machines. These include alloys of amorphous selenium with arsenic and tellurium, zinc oxide, and cadmium sulfide particles in organic binders; one and two layer organic systems; and, most recently, hydrogenated amorphous silicon. The physics of the charge photogeneration and transport processes are examined and illustrated with the above materials. These processes are shown to determine the reproduction characteristics of the xerographic system.