The mature mammalian central nervous system (CNS) has long been considered incapable of significant cell turnover. This view has changed over the last few decades. Recently, the existence of de novo neurogenesis in the adult brain and the presence of stem cells in the mammalian CNS have emerged. The adult brain of both rodents and primates has been shown to embody undifferentiated, mitotically active, precursor cells that are multipotent in nature, and can contribute new, differentiated, neurons and glia to specific regions of the mature brain, such as the olfactory bulb (1–5), the hippocampus (6–8), and the cortex (9–11). While the data clearly suggest the presence of stem cells in the adult CNS in vivo, testing the proliferation, self-renewal, and differentiation capacity of“putative” CNS stem cells relies on the development of methodologies that allow for their isolation and extensive propagation in vitro.