Stability aspects of the Mo/Cu(In,Ga)Se 2 /CdS/ZnO solar cell are reviewed and assessed. These include (i) the chemical stability of the various interfaces present in the device, (ii) the long-term behavior of metastable defects found in the Cu(In,Ga)Se 2 (CIGS) compound, and (iii) the impact of Cu migration on device performance and lifetime. We find that (i) all interfaces within the structure are chemically stable, (ii) metastable defects have a beneficial effect on performance, and (iii) Cu migration effects are reversible and their possible detrimental effects are eclipsed by the beneficial effect of the metastable states. Moreover, Cu out-diffusion from the CIGS layer is absent in photovoltaic-quality CIGS. Finally, we propose a model that explains the exceptional radiation hardness and impurity tolerance of CIGS-based devices, based on the synergetic effect of copper migration and point defect reactions.