Comparison of measured solar-cell parameters with calculations for ideal cells is a powerful tool to assist fundamental understanding and to focus on the most effective fabrication procedures. The emphasis here will be on quantitative separation of individual loss mechanisms in polycrystalline thin-film cells based on CdTe, CuInSe2 (CIS), and related alloys such as CuIn1-xGaxSe2 (CIGS). Several techniques to facilitate separation of losses are described. (C) 2002 Published by Elsevier Science B.V.