Correct toner charging, its sign, magnitude, and distribution, are critical parameters in any electrophotographic development system. This review summarizes recent advances in our understanding of the physics and chemistry governing the toner charging process. These advances include (1) the experimental demonstration that toner charging data are consistent with the theory that toner particles charge up until a material dependent electric field is created at the surface of the toner particle, (2) the suggestion and verification that the surface “work function” of a toner can be expressed as a time dependent surface area weighed sum of the “work functions” of the toner constituents, (3) the experimental and theoretical studies showing that toner charging is not determined by air breakdown, and (4) experimental measurements showing that the toner charge distribution is intrinsically wide, as though a statistical process underlies the charging process, with wrong sign toner resulting from the tail of the distribution.