Charge transfer (CT)-bands, electron trapping, hole trapping, electron release, hole release, metal-to-metal-charge transfer, CT-luminescence, anomalous emission, impurity trapped exciton emission, inter-valence charge transfer, pair-emission, tunneling, photo-electron spectroscopy, redox potentials, photo-ionization, thermal-ionization. All these phenomena deal with the transfer of an electron from one atom in a compound to either another atom in the compound or to the ambient, i.e., outside the compound. The energy needed for, or released in, such transfer carries information on the electron binding energy in the defect levels with respect to the host band levels or the levels in the ambient. First the different types of charge transfer between a lanthanide and the host bands, and how they can be used to construct a host referred binding energy (HRBE) diagram, are reviewed. Then briefly the chemical shift model is introduced in order to convert the HRBE diagram into a vacuum referred binding energy diagram (VRBE). Next charge transfer between transition metal elements and host bands and between Bi3+ and host bands are treated, and finally electron transfer from one defect to another and to the ambient. Illustrating examples are provided.