A summary of recent observations of the properties of ZnS:Cu:Pb and ZnS:Cu:Pb:Mn electroluminescent phosphors is reported. The experiments include studies of the optical and electrical response of these phosphors dispersed in "Lucite" to sudden changes in a constant applied field, as well as microscopic studies of the luminescence of individual particles. Both green and yellow phosphors were studied. The most important results observed primarily on green phosphors are the following: Slow changes are found to take place in a phosphor in any given constant electric field, but a steady state is finally approached. A sudden departure from this state produced by changing the applied field will in general produce very little light compared to that produced on returning promptly to the conditions under which the steady state was established. Further, the luminescence in alternating electric fields of individual grains is extremely inhomogeneous, being restricted to small "pinpoints" or "spots" very much smaller than the individual particles. The spots studied luminesced only once per cycle.It is concluded that excitation of luminescent centers occurs primarily by ionization, with recombination and radiation occurring when subsequent electrical changes make this possible. Although excitation of luminescent centers to bound states followed by prompt radiation does occur, under the conditions of these experiments this process is of secondary importance in producing light in all the phosphors studied, and is quite negligible in the green phosphors. These and other results are discussed in connection with the several possible mechanisms by which electric fields may bring about the excitation of luminescent centers.