Numerical modeling is increasingly used to obtain insight in to the details of the physical operation of thin-film solar cells. Over the years several modeling tools specific to thin-film PV devices have been developed. A number of these tools have reached a mature status and are available to the PV community. Some of the most commonly used programs are presented and the possibilities as well as the shortcomings are discussed. Also, for the different thin-film PV devices (CdTe, CIGS, and, to a lesser extent, amorphous silicon and nano-structured solar cells) an overview is given of modeling efforts and achievements. Copyright © 2004 John Wiley & Sons, Ltd.