Neural stem cells are the source of all neurons, astrocytes and oligodendrocytes of the central nervous system. While the vast majority of neural stem cells are consumed during embryonic development, a subpopulation persists in specialized regions of the adult mammalian brain where addition of cells, notably neurons, -continues throughout life. The significance and physiological role of adult neurogenesis are still debated but it is generally believed that neural stem cells may be used to establish novel therapies for certain neural pathologies. In this chapter we describe the main features of neural stem cells during embryonic development and adulthood as well as the key mechanisms known to influence their proliferation versus differentiation. We then discuss the current views on the function of adult neurogenesis and the first attempts to use neural stem cells in therapy. Since the focus of this book is on regenerative medicine, we will mainly describe neural stem cells of mammalian organisms and briefly mention studies on other phyla only if particularly relevant.