The various stages in excitation of luminescence by cathode rays are examined with the object of assessing the possible efficiency of energy conversion. It is shown that reflection of primary electrons constitutes a major loss of energy. The elastic scattering process responsible is dependent on coulomb interaction between the electrons and the atoms of the solid. Energy losses inside the phosphor due to ionization follow a fairly simple relation to residual electron energy. At all but low electron energies the relation yields a modified Thomson-Whiddington range energy law. After cascade production of secondary electrons a relatively uniform volume of excitation results, bounded in most cases by the phosphor surface where loss of electrons occurs as secondary emission. Secondary electrons incapable of further ionization will undergo phonon interaction with further energy loss to the luminescence process. At thermal velocities they will be available for radiative recombination. Approximate theoretical conversion efficiencies should be in the region of 25%. An experimental estimate for ZnS-Ag phosphors gives 23% which is in close agreement.The relations, found experimentally for microcrystalline phosphor layers, between luminescence, primary electron energy and beam current density can only be qualitatively related to the detailed processes of excitation. However, more accurate interpretations are possible for data obtained by use of single crystals.