This contribution is a summary of an international, interdisciplinary workshop dedicated to defects in chalcopyrite semiconductors and their relation to the device characteristics of thin-®lm solar cells, held on 3±5 June 1996 in Oberstdorf, Germany. Results of dierent characterization methods were brought together to identify common observations. The comparison of results from electrical defect spectroscopy and luminescence investigations con®rmed the presence of energetic distributions of defects throughout the bandgap of chalcopyrite thin ®lms. Electrical defect spectroscopy detects a defect about 280 meV above the valence band edge of Cu(In, Ga)Se 2 regardless of the preparation conditions of the sample. In a solar cell the density of this defect depends on the operation conditions. This observation might be related to the migration of copper in an electric ®eld, which occurs even at room temperature. Other defects appear to be related to processing or impurities. Photoluminescence decay measurements yield time constants of several nanoseconds under low injection conditions. Modelling of the current±voltage characteristics of Cu(In, Ga)Se 2 -based thin-®lm cells suggests that compensating acceptor states in the CdS or at the heterointerface are responsible for the frequently observed cross-overs between the dark and illuminated curves.