The dogma that solid tumors are composed of tumor cells that all share the same ability to produce proliferating daughter cells has been challenged in recent years. There is growing evidence that many adult tissues contain a set of tissue stem cells, which might undergo malignant transformation while retaining their stem cell characteristics. These include the ability of indefinite self-renewal and the capability to differentiate into daughter cells of tissue-specific lineages. Brain tumors such as medulloblastomas or glioblastomas often contain areas of divergent differentiation, which raises the intriguing question of whether these tumors could derive from neural stem cells (NSCs).This chapter reviews the current knowledge of NSCs and relates them to brain tumor pathology. Current therapy protocols for malignant brain tumors are targeted toward the reduction of bulk tumor mass. The concept of brain-tumor stem cells could provide new insights for future therapies, if the capacity for self-renewal of tumor cells and growth of the tumor mass would reside within a small subset of cancer cells.