The mammalian target of rapamycin (mTOR) regulates cell growth in response to various intracellular and extracellular signals. It assembles into two multiprotein complexes: the rapamycin-sensitive mTOR complex 1 (mTORC1) and the rapamycin-insensitive mTORC2. In this study, we inactivated mTORC1 in mice by deleting the gene encoding raptor in the progenitors of the developing CNS. Mice are born but never feed and die within a few hours. The brains deficient for raptor show a microcephaly starting at E17.5 that is the consequence of a reduced cell number and cell size. Changes in cell cycle length during late cortical development and increased cell death both contribute to the reduction in cell number. Neurospheres derived from raptor-deficient brains are smaller, and differentiation of neural progenitors into glia but not into neurons is inhibited. The differentiation defect is paralleled by decreased Stat3 signaling, which is a target of mTORC1 and has been implicated in gliogenesis. Together, our results show that postnatal survival, overall brain growth, and specific aspects of brain development critically depend on mTORC1 function.