Medulloblastoma is a malignant embryonal tumor predominantly of childhood. It occurs principally in the midline cerebellar region but is prone to invade the meninges and cerebrospinal fluid spaces. It is the principal posterior fossa tumor in children and represents 20% of all brain tumors in this age group. Considerable advances have been made in characterizing the morphologic subtypes, immunophenotypes, and in vitro behavior of cell lines. However, the cell of origin and its relationship to other related primitive neuroectodermal tumors remains uncertain. Although recent advances in the application of proliferation markers and the use of apoptosis in situ labelling have been applied, a prognostically relevant marker applicable to surgical neuropathology remains to be developed. The fundamental molecular pathogenesis of tumor development remains unknown, although recent exciting advances have shown alterations in some members of the PAX family of genes as well as increased expression of neurotrophin receptors. These may prove to be of prognostic significance.