Most descriptions treat the cerebellum as a uniform structure, and the possibility of important regional heterogeneities in either chemistry or physiology is rarely considered. However, it is now clear that such an assumption is inappropriate. Instead, there is substantial evidence that the cerebellum is composed of hundreds of distinct modules, each with a precise pattern of inputs and outputs, and expressing a range of molecular signatures. By screening a monoclonal antibody library against cerebellar polypeptides we have identified antigens--zebrins--that reveal some of the cerebellum's covert heterogeneity. This article reviews some of these findings, relates them to the patterns of afferent connectivity, and considers some possible mechanisms through which the modular organization may arise.