CdS/ Cu x S thin film solar cells are one of the most investigated photovoltaic devices. After extensive fundamental and technological studies, efficiencies as high as 10% have actually been achieved with Cd 1−y Zn y S/ Cu 2 S cells. However, many problems must be solved before industrial production can start. Indeed, there is great competition between different preparation methods and different cell structures. These cells can be realized in the form of front-wall or back-wall cells. The CdS layers can be prepared by thermal evaporation, sputtering, spraying, sintering etc. The Cu x S cells films may be obtained by a wet or a dry process. The properties of the cells depend markedly on the preparation method; this applies especially to the density of interfacial states at the junction and to the presence of trap levels in the CdS space charge region. Hole traps are particularly active, as they can modulate the barrier when the junction is illuminated with radiation, thereby producing photoholes in the space charge region of CdS. This modulation of the barrier has several consequences such as the crossing of the dark and light current-voltage curves, photocapacitance effects and photodetection properties. A competitive industrial product will be obtained when acceptable solutions are found to the problems of the long-term stability, the passivation of deep levels and interfacial states and the energy payback of the cells.