Researchers often think of nucleosomes as inert and static structures. But the laws of physical chemistry—which apply to any molecular complex—remind us that this picture cannot be correct. Indeed, many early studies revealed that nucleosomes undergo a complex set of assembly/disassembly processes, and more recent studies reveal that nucleosomes also have remarkable dynamic behavior. These properties of nucleosomes have practical consequences for experiments carried out in vitro, and they have important consequences for nucleosome function in vivo. This article will not discuss specific experimental methods; rather, it will summarize several aspects of the equilibrium and dynamic behavior of real nucleosomes that are of particular importance for studies of nucleosome structure and function.