The stability of CdTe/CdS solar cells depends on spatial changes of defects and impurities throughout the cell. Degradation effects are often associated with metal diffusion from the back contact of the cell, which is Cu in most cases. However, cells with stable back contact can also exhibit instability, as also all the other cell layers are potential sources of impurities causing instability. Cell degradation due to generation of defects from external influences like particle irradiation, e.g. in space, is another reason for instability. The development of stable back contacts as well as sources of instability in the cell performance are discussed with a special focus on the US layer and CdTe/CdS interface, which are very sensitive to the accumulation of impurities and defects. A non-destructive method to assess the stability issue is described. An analysis of performance stability with respect to defect generation caused by high-energy protons and electrons is presented. Additionally, effects of meta-stability and the capability to recover from degradation by defect relaxation are shown. (C) 2003 Elsevier B.V. All rights reserved.