It is a major challenge to understand how the neuroepithelial cells of the developing CNS choose between alternative cell fates to generate cell diversity. In invertebrates such as Drosophila melanogaster and Caenorhabditis elegans, asymmetric segregation of cell-fate determining proteins or mRNAs to the two daughter cells during precursor cell division plays a crucial part in cell diversification. There is increasing evidence that this mechanism also operates in vertebrate neural development and that Numb proteins, which function as cell-fate determinants during Drosophila development, may also function in this way in vertebrates. Recent studies on mouse cortical progenitor cells have provided the strongest evidence yet that this is the case. Here, we review these and other findings that suggest an important role for the asymmetric segregation of Numb proteins in vertebrate neural development.