Neural stem cells are present not only in the developing nervous systems, but also in the adult central nervous system of mammals, including humans. The mature central nervous system has been traditionally regarded as an unfavorable environment for the regeneration of damaged axons of mature neurons and the generation of new neurons. In the adult central nervous system, however, newly generated neurons from adult neural stem cells in specific regions exhibit a striking ability to migrate, send out long axonal and dendritic projections, integrate into pre-existing neuronal circuits, and contribute to normal brain functions. Adult stem cells with potential neural capacity recently have been isolated from various neural and nonneural sources. Rapid advances in the stem cell biology have raised exciting possibilities of replacing damaged or lost neurons by activation of endogenous neural stem cells and/or transplantation of in vitro-expanded stem cells and/or their neuronal progeny. Before the full potential of adult stem cells can be realized for regenerative medicine, we need to identify the sources of stem cells, to understand mechanisms regulating their proliferation, fate specification, and, most importantly in the case of neuronal lineages, to characterize their functional properties. Equally important, we need to understand the neural development processes in the normal and diseased adult central nervous system environment, which is quite different from the embryonic central nervous system, where neural development has been traditionally investigated. Here we will review some recent progress of adult neural stem cell research that is applicable to developmental neurobiology and also has potential implications in clinical neuroscience.