When eukaryotic genes become transcriptionally active, the structure of the chromatin in their neighborhood changes to accommodate transcription factors and the passage of RNA polymerase. It is now clear that this process requires not only the transcription factors, but also cooperation with histones and with cofactors that help to remodel or displace nucleosomes. The most intense interest has focused quite naturally on the organization of nucleosomes in the neighborhood of promoters and enhancers. When the genes they control are active, these regions are marked by increased accessibility to a variety of enzymic and chemical probes. This typically signals a disrupted or missing nucleosome, and one can ask what it takes to effect this rearrangement. But there are also higher levels of chromatin organization, and in other situations these too must be unravelled, attended by their own activation mechanisms.