We review here recent data that have brought into sharper focus a number of important biological properties of the neoplastic cells in childhood primitive neuroectodermal tumors (PNETs) of the central nervous system (CNS). Studies of this group of tumors, as exemplified by posterior fossa medulloblastomas (MBs), suggest that neoplastic cells in PNETs partially recapitulate stages in the maturation of normal human neuroblasts. These findings may contribute to the elucidation of the mechanisms involved in tumor initiation and progression because oncogenes and antioncogenes appear to exert their effects in a cell type-specific manner that also depends on the maturational state of a given cell. Currently, a large body of data suggests that populations of cells in PNETs (e.g., MBs) exhibit one or more molecular defects in the sequence of maturational events leading to the exit of stem cells or partially committed neuron-like precursors from the cell cycle, followed by their terminal differentiation into neurons. This, together with the orchestrated interactions of as yet unidentified oncogenes and antioncogenes in these PNET cells, may represent a cluster of molecular abnormalities that underly the emergence of the highly malignant phenotype that characterizes childhood PNETs.