The development of the cerebellum occurs in four basic steps. During the first epoch, genes that mark the cerebellar territory are expressed in a restricted pattern along the anterioposterior axis of the embryo. In the second, an embryonic region termed the rhombic lip generates precursors of the granule cell population of the cerebellar cortex, and the lateral pontine nucleus and olivary nucleus of the brain stem. In the third period, the program of neurogenesis of the granule neuron gives rise to the formation of the fundamental layers of the cerebellum and to the pattern of foliation. Concomitantly, programs of gene expression define the principal neuronal classes, the granule cell and Purkinje cell, that will establish the cerebellar circuitry in the postnatal period. Understanding the molecular mechanisms underlying these steps of development is likely to yield important insights into malformations such as Joubert syndrome.