Neural stem cells, with the capacity to self renew and produce the major cell types of the brain, exist in the developing and adult rodent central nervous system (CNS). Their exact function and distribution is currently being assessed, but they represent an interesting cell population, which may be used to study factors important for the differentiation of neurons, astrocytes and oligodendrocytes. Recent evidence suggests that neural stem cells may also exist in both the developing and adult human CNS. These cells can be grown in vitro for long periods of time while retaining the potential to differentiate into nervous tissue. Significantly, many neurons can be produced from a limited number of starting cells, raising the possibility of cell replacement therapy for a wide range of neurological disorders. This review summarises this fascinating and growing field of neurobiology, with a particular focus on human tissues.