Cu(In,Ga)Se-2 (CIGS) based photovoltaic cells have demonstrated the highest solar energy conversion efficiencies ever for thin film devices. They also exhibit excellent stability in field tests and exceptional radiation hardness. The apparent paradox is that these results are obtained with a cell that contains a material that is chemically the most complex of the materials used in the various thin film solar cells. Moreover, the device itself contains many elements, compounds and interfaces, all potential focus for evolution or reaction. Because of their central importance, the basic scientific foundations for the remarkable lifetime and stability of those devices are discussed, especially but not exclusively from a chemical point of view. A first section is devoted to the assessment of the intrinsic thermodynamic stability of CIGS by a critical evaluation of available data. Its relationship with the formation energy of point defects is stressed. The chemical stability of the device interfaces are examined, including prospective buffer and window layers.