During the development of the embryo, many of the important developmental decisions that are made by individual cells depend on molecular cues that are transiently present but that vanish as development proceeds. Yet, the commitment of a cell to a developmental pathway is maintained in its cellular progeny. Many kinds of molecular mechanisms can be imagined to preserve such a state of determination, based, for example, on self-maintaining intracellular or intercellular feedback loops, but evidence has been accumulating for a type of mechanism that relies on the establishment of self-perpetuating alternative chromatin states that determine the activity of broad chromatin domains. In Drosophila, this type of mechanism involves the products of the Polycomb group of genes (PcG) and is best known for its role in maintaining patterns of homeotic gene expression. The effects of the PcG proteins in establishing a cellularly inherited but variable repressed state are reminiscent of position-effect variegation in Drosophila, silencing in yeast, and paramutation in plants. The study of the mechanisms regulating the expression of homeotic genes has provided much of the current knowledge of PcG proteins and their action.