Due to the complexity of the NSC niche organization, the lack of specific NSC markers and the difficulty of long-term tracking these cells and their progeny in vivo the functional properties of the endogenous NSCs remain largely unexplored. These limitations have led to the development of methodologies to efficiently isolate, expand, and differentiate NSCs ex vivo. We describe here the peculiarities of the neurosphere assay (NSA) as a methodology that allows to efficiently isolate, expand, and differentiate somatic NSCs derived from the adult forebrain periventricular region while preserving proliferation, self-renewal, and multipotency, the main attributes that provide their functional identification.