The ventricular zone – a monolayer of cells lining the fourth ventricle on the ventral surface of the cerebellar anlage, and the rhombic lip – a transient structure in the most posterior part of the cerebellar anlage that forms the interface between the neural tube and non-neural roofplate ectoderm. The rhombic lip gives rise to the entire complement of glutamatergic neurons that populate the IGL. The first glutamatergic neurons born are the projection neurons of the deep cerebellar nuclei. In the mouse these arise between embryonic day 10.5 and E12.5, and migrate along the sub pial stream to the rostral end of the developing cerebellum. From E12.5 onwards, the rhombic lip generates granule ABT-263 progenitor cells and unipolar brush cells. Exiting the rhombic lip, GPCs migrate rostrally across the pial surface of the cerebellum to form a secondary germinal zone, the external germinal layer, which covers the pial surface of the cerebellum. This cell layer proliferates extensively until the second postnatal week, producing a vast number of mature granule cells, which become post-mitotic within the EGL before migrating radially along Bergmann glial fibres into the IGL, a process that is complete by postnatal day 21. Distinct from the rhombic lip, the ventricular zone gives rise to all cerebellar cells of the c-aminobutyric acid ergic, and glial lineages. The first of these, the Purkinje cells, are born from E10.5 then migrate radially towards the pial surface of the cerebellum and settle as a distinct monolayer of cells around the time of birth. Closely following this the Bergmann glia are generated and migrate radially behind the developing Purkinje cell population before undergoing morphological maturation postnatally. Interneurons and the remaining glial population are then generated in a sequential manner. These cell types are derived from progenitors that delaminate from the ventricular zone and continue to divide in the WM. These tightly coordinated developmental processes rely on the spatio-temporal specific activity of several key signalling pathways. The sonic hedgehog signalling pathway, for example, is the main mitogenic factor driving GPC proliferation within the EGL. The Wnt/b-catenin signalling pathway has been shown to play an important part in regulation of neural stem and progenitor populations within the central nervous system, but its role in cerebellum development is only partially defined. Wnt1 is an important regulator of early cerebellum development.