The discovery of new neurons in adult mammalian brain challenged the concept of the adult vertebrate central nervous system (CNS) as a hard-wired immutable system. Following the description of neural stem cells (NSCs) in human brain during the last decade, increasing interest for their use as therapeutic agents has emerged. NSCs are semiquiescent cells, maintained in particular niches in an undifferentiated state, and capable of self-renewal and differentiation into the three main neural lineages: neurons, astrocytes, and oligodendrocytes. Due to their multipotent capacity, a connection between NSCs and brain tumors has been suggested. Brain tumors, in particular gliomas, consist of a heterogeneous population of cells at multiple stages of differentiation. A particular subpopulation of brain tumor cells present characteristics that are similar to those of NSCs. For this reason, these cells have come to be known as brain tumor stem cells (BTSCs). BTSCs are thought to be responsible for maintaining the tumor bulk and for the recurrence of brain tumors after surgical resection. The origin of BTSCs is still largely unknown; however, a growing body of evidence suggests the malignant transformation of NSCs as a plausible hypothesis. Here we review the current findings regarding the relation between NSCs and BTSCs, from in vitro studies to clinical findings.