Polycomb group (PcG) and trithorax group (trxG) proteins form molecular modules of a cellular memory mechanism that maintains gene expression states established by other regulators. In general, PcG proteins are responsible for maintaining a repressed expression state, whereas trxG proteins act in opposition to maintain an active expression state. This mechanism, first discovered in Drosophila and subsequently in mammals, has more recently been studied in plants. The characterization of several Polycomb Repressive Complex 2 (PRC2) components in Arabidopsis thaliana constituted a first breakthrough, revealing key roles of PcG proteins in the control of crucial plant developmental processes. Interestingly, the recent identification of plant homologues of the Drosophila trithorax protein suggests a conservation of both the PcG and trxG gene regulatory system in plants. Here, we review the current evidence for the role of PcG and trxG proteins in the control of plant development, their biochemical functions, their interplay in maintaining stable expression states of their target genes, and point out future directions which may help our understanding of PcG and trxG function in plants.