The nervous system is so complex that it is not ordinarily considered as an interesting model for studying cellular mechanisms. However, its morphology (at least in some well-defined areas) provides essential information because cells are polarised: cell body, axons and dendrites can be easily identified in vivo, an essential property when studies are concerned with the intracellular traffic of molecules and with the biochemical steps involved in cell adhesion and cell recognition mechanisms. The development of the central nervous system presents features that are phylogenetically preserved. Although most studies were performed on a special portion of the central nervous system (CNS), the cerebellum (Fig.1), because its organisation is extremely simple, reproducible from one area to the other and its development is relatively synchronous compared with other brain areas, the mechanisms deduced from this simple part of the CNS can be extrapolated to other brain areas. The formation of the specialised contacts between neurones at the synapses is only one aspect of the complexity of the CNS development. In fact, the formation of the CNS can be reduced to sequential steps: neuronal cell proliferation, neuronal cell migration and axonal growth, synaptogenesis and glial wrapping (myelination and astrocytic insulation). In the cerebellum, after the initial phase of neuronal cell proliferation, the cell bodies of immature neurones undergo migration on pre-existing astrocytic fibres to reach their adult location.