The adult vertebrate central nervous system (CNS) consists of four major differentiated cell types: neurons, astrocytes, oligodendrocytes and ependymal cells. Historically, there has been a disagreement on how these differentiated cell types are generated in the CNS. Progress remains hindered by the complexity of cell structure in this system, the lack of specific cell surface markers to identify distinct cell types and the presence of numerous transit amplifying cell populations that rapidly generate early progenitors. At present, it is clear that some cells, termed neural stem cells, can generate neurons as well as astrocytes and oligodendrocytes of the glial lineage both in vitro and in vivo. Additionally, specific neural stem cell populations have also exhibited tissue lineage plasticity.