Pattern formation has fascinated biologists since the time of Aristotle, but only recently have new tools begun to reveal the underlying mechanisms that create these patterns during development. In particular, the central nervous system is dynamically patterned and highly modular, ranging from nuclear cell clusters in the brain stem and spinal cord to the elaborate cytoarchitecture of the neocortex. Similar developmental processes divide brain structures such as the cerebral cortex, basal ganglia, superior colliculus, and cerebellum into these sub-compartments. The way neural modules form and the mechanisms that establish connectivity between these modules is one of the most complex problems in neuroscience and also one of the most important. This monograph focuses on pattern formation in the developing cerebellum.