The paper presents current views concerning the role of stem cells in neoplasia, with particular emphasis of CNS tumors. First, some arguments are presented supporting the thesis that at the first stage of neoplasia, the cellular target for carcinogens are normal stem cells or progenitor cells. Also, discussed are important problems associated with attempts at identification of cellular sources of neoplasms. One of these is the difficulty encountered with distinguishing stem cells and non-differentiated cells. Second, data are presented allowing the conclusion that within neoplastic tissue exist neoplastic stem cells – cells enabling regeneration of this pathological tissue. In the context of discussion concerning the role of stem cells in the pathogenesis of neoplasms, a new theory about physiologic and pathologic role of normal and damaged proteins, e.g. APC, SUFU, EGFR, c-MYC, P53 is presented. Discussed is also the role of proteins controlling modification of chromatin, e.g. the Polycomb protein. These proteins are extremely important for the differentiation process. The paper presents also own preliminary experience with the role of stem cells in the pathogenesis of CNS tumors. Finally, presented are premises providing hope for application of knowledge concerning neoplastic stem cells in designing novel modalities of oncologic therapies. Development of such therapies may be based on the search for chemotherapeutic agents which would selectively eliminate neoplastic stem cells. It is also possible to use normal but genetically modified stem cells to detect neoplastic stem cells and to eliminate them.