This chapter reviews the properties of zincblende CdS/ZnS and (Cd,Zn)S/ZnS quantum wells grown on GaAs substrates. The properties discussed are band-gap energy, exciton binding energy and lattice constant. Electronic states in quantum wells are described. Photoluminescence (PL) in the deep-blue/ultraviolet spectral region is observed from CdS/ZnS quantum well (QW) structures with narrow CdS wells strained to the ZnS lattice constants. The calculated exciton transition energies account for the strain in the CdS layer and use a variational model for the exciton binding energy. The quenching of the excitonic PL for increasing lattice temperature is suppressed when the quantum wells are incorporated in seperate confinement heterostructures. Normalized photoluminescence spectra of three CdS/ZnS single quantum wells (SQWs) with different well widths excited below the ZnS band gap, and experimental and theoretical exciton transition energies for CdS/ZnS QWs on ZnS buffer are illustrated. Illustrations also include excitonic exchange splitting in two CdS/ZnS SQWs as a function of excitation (resonant PL) or detection (PLE) energy relative to the PL maximum, resonant PL spectra of a 1.2 ML CdS/ZnS SQW for various resonant excitation energies, and µ-PL spectra from etched squared mesas of different base length prepared from a 2.7 ML thick CdS/ZnS SQW. Biexcitonic photoluminescence and two-photon absorption are observed in (Cd,Zn)S/ZnS multipe quantum wells (MQW). A binding energy of 38 meV is reported for the confined biexciton in ultrathin CdS/ZnS QWs. Ultraviolet stimulated emission and optical gain with strong signatures of electron-hole correlation up to room temperature is observed in (Cd,Zn)S/ZnS MQW structures.