Granule cells and Purkinje cells are the major populations of neurons in the cerebellum. Their specification depends on a combination of regional identity and spatiotemporal cues. These are conferred by patterning systems in the early embryo that determine anteroposterior and dorsoventral positional coordinates and an age-dependent signal (or signals) whose nature is obscure. While a number of important questions remain about the nature of cerebellar progenitor pools and their precise boundaries, a variety of fate-mapping and genetic approaches have indicated that both granule cells and Purkinje cells arise from different dorsoventral domains within hindbrain rhombomere 1. Unusually, granule cell precursors undergo a subsequent transit amplification stage regulated by Purkinje cell signals, within a transient superficial germinal layer. Recent evolutionary insights suggest that this phase of Sonic hedgehogdependent transit amplification is only found in amniotes. Evolutionarily, since secondary proliferation arose independently of granule cell specification, it is likely to be an adaptation purely for postspecification regulation of granule cell numbers.