The discovery of stem cell populations in the adult central nervous system (CNS) that continually produce neurons and glial cells, and the hypothesis that they could contribute to neural plasticity/repair, has opened new and exciting areas of research in basic cell biology and regenerative medicine. The success of these studies relies on understanding the functional features and the normal fate of neural stem cells (NSCs) in vivo as well on the development of in vitro culture conditions enabling isolation, extensive propagation, and rigorous characterization of the "putative" NSCs. The neurosphere assay (NSA) has emerged as a valuable tool for isolating embryonic and adult CNS stem cells and for studying their biology. However, because this assay may select and expand a heterogeneous stem/progenitor cell population, rigorous clonal and serial subcloning analyses are required to detect and document stem cell activity and to unequivocally identify bona fide stem cells. We illustrate and discuss methods for the isolation, propagation, cryopreservation, and functional characterization of NSCs, focusing on the essential issue of their clonogenic capacity.