Here, we discuss the identity, heterogeneity and functions of radial glial cells mostly in the developing central nervous system (CNS). First, we define radial glial cells by morphological, cell biological and molecular criteria as true glial cells, akin to astroglia. We then describe the appearance of radial glial cells during neural development as a precursor intermediate between immature neuroepithelial cells and differentiating progeny. Then we review the diverse progeny arising in different lineages from radial glial cells as observed by clonal analyses and time-lapse imaging. This leads us to discuss the molecular mechanisms involved in the regulation of the lineage heterogeneity of radial glial cells - including their diversity in distinct regions of the CNS. We conclude by considering the possible mechanisms allowing neurogenic radial glial cells to persist into adulthood in various vertebrate classes ranging from fish to birds, while neurogenic glial cells become restricted to few small regions of the adult forebrain in mice and men.