Efficiencies of CuIn1-xGaxSe2-ySy (CIGSS) modules are comparable to those of lower end crystalline-Si modules. CIGSS layers are prepared by reactive co-evaporation, selenization/sulfurization of metallic or compound precursors, reactive co-sputtering and non-vacuum techniques. CuIn1-xGaxS2 (CIGS2) layers are prepared by sulfurization of Cu-rich metallic precursors and etching of excess Cu2-xS. Usually heterojunction partner CdS and transparent-conducting bilayer ZnO/ZnO:Al layers are deposited by chemical bath deposition (CBD) or RF magnetron sputtering. CIGSS solar cell efficiencies have been improved by optimizing Cu, Ga and S proportions and providing a minute amount of Na. This paper reviews preparation and efficiency improvement techniques for CIGSS solar cells. (c) 2006 Elsevier B.V. All rights reserved.