In the adult mammalian brain, bona fide neural stem cells were discovered in the subventricular zone (SVZ), the largest neurogenic niche lining the striatal wall of the lateral ventricles of the brain. In this region resides a subpopulation of astrocytes that express the glial fibrillary acidic protein (GFAP), nestin and carbohydrate Lewis X (LeX). Astonishingly, these GFAP-expressing progenitors display stem-cell-like features both in vivo and in vitro. Throughout life SVZ astrocytes give rise to interneurons and oligodendrocyte precursors, which populate the olfactory bulb and the white matter, respectively. The role of the progenies of SVZ astrocytes has not been fully elucidated, but some evidence indicates that the new neurons play a role in olfactory discrimination, whereas oligodendrocytes contribute to myelinate white matter tracts. In this chapter, we describe the astrocytic nature of adult neural stem cells, their organization into the SVZ and some of their molecular and genetic characteristics.