The purpose of this book is to advance the understanding of SrS-based a. c. thin film electroluminescent (ACTFEL) devices. The role of traps in the bulk phosphor layer in the light emission mechanism from ACTFEL devices is studied, characterized and modeled. Experiments were performed to observe the response of the ACTFEL devices to tailored voltage excitations. A physical model was developed to describe the opto- electronic processes taking place in the phosphor; analytical equations were written and numerically simulated to plot the flux and luminance responses of the device to similarly tailored voltage excitations. The voltage excitation parameters such as amplitude, rise times and fall times were varied both experimentally and in simulations and their effect on the opto-electronic response of the device was studied. Thermally stimulated luminance studies were performed to determine critical device parameters. Theoretical predictions matched the experimental data in a qualitative manner. A much improved quantitative accuracy is obtained when the role of dipoles in the EL mechanisms is incorporated into the model.