In contrast to the haematopoietic system in which each cell type is subject to constant turnover, thus endowing this system with the permanent ability to reconstitute itself, the nervous system has long been known as an organ devoid of spontaneous cellular reconstitution. Yet the discovery that certain regions of the mammalian central nervous system do sustain neurogenesis throughout life, together with the fact that cells can be isolated from the adult brain that generate neurons in vitro, has led to the idea that the nervous tissue harbours neural stem cells. The term "neural stem cell" has now become associated with enormous expectations for curing diseases of the nervous system. Yet many of the biological fundamentals of neural stem cells need to be revealed before these expectations can be properly judged or even fulfilled. This begins with the question of whether the neural stem cell corresponds to a real entity or rather represents an in vitro dedifferentiation phenomenon. In this chapter we attempt to give an overview of our current knowledge of the biology of the presumable adult neural stem cell. This is followed by a comparative assessment of the possibilities of using adult neural stem cells and embryonic stem cells for therapeutic approaches in the context of neurodegenerative diseases. Finally, we will look at the "evil side" of stemness by discussing the evidence that brain cancers may originate from cells with stem cell-like properties.