History and current status of Cu-ternary-based photovoltaic devices are reviewed. Heterojunction and homojunction research on CuInS2, CuInSe2 and CuInTe2 is covered. Some emphasis is placed on the CdS, Cd(Zn)S/CuInSe2 thin-film solar cell, which has reached a 10% solar-conversion efficiency and has demonstrated remarkable stability characteristics. Fundamental properties of interface formation are presented, by using surface analysis (AES, angular-resolved XPS, EELS) techniques. The possible formation of binary semiconductor compounds (e.g. Cu x S and Cu x Se) during the initial stages of heterojunction formation is reported. The evaluation of heterojunction response is studied by using high-resolution electron-beam-induced current (EBIC) investigations on fractured solar-cell cross-sections. Schottky barrier and homojunction responses are reported for present solar-cell designs. The importance and effects of postdeposition heat treatments on the performance of these thin-film cells are discussed. In particular, complementary EBIC and SIMS data are used to ascertain the necessity of the oxygen/temperature treatments of this device. Compositional changes in interface and layer properties upon temperature stress that affect device reliability are reported. The present design and processing of the Cu ternary devices are summarized and the future directions of research and largescale processing of such solar cells are indicated.