Studies on the electro-optical properties of CdS-type materials have gained much importance due to their wide applications as sensitive photoconductors, IR detectors, solar cells, lamp phosphors etc. Incorporation of rare earth ions into these materials improves their photosensitivity at least a thousand fold. This book presents the electro-optical studies of rare earth doped (Cd-Zn)S films prepared by chemical deposition technique. The theories used in understanding the phenomenon of photoconductivity and photoluminescence have been elaborately discussed. Techniques of film preparation and the various measuring arrangements used have also been described. The optical properties as well as the structure and morphology of the films have been extensively discussed and the optimum composition and preparatory conditions for improving the quality of the films have been suggested. It is hoped that the different studies and their observations would be useful for interested researchers in this field.