Chromatin is increasingly recognized as a highly dynamic entity. Chromosome sites in lower and higher eukaryotes undergo frequent, rapid, and constrained local motion and occasional slow, long-range movements. Recent observations have revealed some of the functional relevance of chromatin mobility. Paradoxically, both the mobility and immobility of chromatin appear to have functional consequences: Local diffusional motion of chromatin is important in gene regulation, but global chromatin immobility plays a key role in maintenance of genomic stability.