The Polycomb-group proteins (Pc-G) are transcriptional repressors that control many key developmental transitions in plants. Recent whole genome profiling of the histone methylation marks that are characteristic of the Pc-G have suggested many novel targets of Pc-G regulation. However, the number and nature of targets also suggests that Pc-G regulation may be much more dynamic than hitherto assumed. We discuss the role of histone methylation in gene regulation, the complexes involved in setting and reading the mark and how activity states may be reset during development.