Polycomb group (PcG) proteins are transcriptional repressors that control expression of developmental regulator genes in animals and plants. Recent advances in our understanding of the PcG system include biochemical purifications that revealed a substantial variety in PcG complex composition. These different complexes contain distinct chromatin-modifying activities and engage in cross-talk with other chromatin modifications. Complementing these biochemical analyses, structural studies have begun to provide insight into how PcG proteins interact with each other and with chromatin. Finally, genome-wide binding profiling and the ensuing functional analysis of target gene regulation revealed that the PcG system is not only used for the permanent silencing of developmental regulator genes. Rather, PcG mediated repression also constitutes a mechanism for dynamic control of gene transcription.