Recent work has begun to identify neural stem and progenitor cells in the embryonic and adult brain, and is unravelling the mechanisms whereby new nerve cells are created and delivered to their correct locations. Radial glial (RG) cells, which are present in the developing mammalian brain, have been proposed to be neural stem cells because they produce multiple cell types. Furthermore, time-lapse imaging demonstrates that RG cells undergo asymmetric self-renewing divisions to produce immature neurons that migrate along their parent radial fibre to reach the developing cerebral cortex. RG cells also produce intermediate progenitor (IP) cells that undergo symmetric division in the subventricular zone of the embryonic cortex to produce pairs of neurons. The symmetric IP divisions increase cell number within the same cortical layer. This two-step process of neurogenesis suggests new mechanisms for the generation of cell diversity and cell number in the developing cortex and supports a model similar to that proposed for the development of the fruit fly CNS. In this model, a temporal sequence of gene expression changes in asymmetrically dividing self-renewed RG cells could lead to the differential inheritance of cell identity genes in cortical cells generated at different cell cycles.