In the adult brain, new neurons develop from neuronal stem and progenitor cells. Although these precursor cells presumably exist in the entire brain, only in the hippocampus and the olfactory system new neurons are continuously generated from these cells. Adult neurogenesis is regulated in a complex manner and dependend from "activity", which argues in favor of the idea that adult neurogenesis is functionally relevant. First theories address the putative functional significance of neuronal stem cells in both neurogenic and non-neurogenic regions. Finally, first ideas have emerged on what happens if the normal regulation of neuronal stem cell activity is disturbed.