It is generally assumed that neurogenesis in the central nervous system ceases before or soon after birth. In the last three decades, however, several studies have reported that new neurons continue to be added into the brain of adult fish, frogs, reptiles, birds and mammals. The precursor cells that give rise to the neurons generated in adulthood are generally located in the walls of the brain ventricles. From these proliferative regions, neuronal precursors migrate toward their final targets where they differentiate; they often traverse long distances through complex brain parenchyma. The identity of the neuronal precursors in the brains of adult animals is still unknown. Experiments in adult birds suggest that proliferating radial cells may be the neuronal precursors. In adult mice, cells present in the sub-ventricular zone can generate neurons in vivo and in vitro. These neuronal precursors can be induced to proliferate in vitro when exposed to growth factors and retain their potential to differentiate into neurons and glia. Whether these putative neural stem cells can differentiate into multiple neuronal types remains to be determined. The neuronal precursors of the adult brain could be used as a source of cells for neuronal transplantation. In addition, these cells could be manipulated in vivo or in vitro to introduce genes into the brain. Adult neurogenesis offers new experimental opportunities to study neuronal birth, migration and differentiation and for the treatment of neurological diseases.