Numerous histologic subtypes of medulloblastoma have been described. Classic medulloblastoma (undifferentiated) consists of sheets of small round blue cells as shown in this image. Embryonal tumors comprise about 20% of pediatric brain tumors. Medulloblastoma is the most commonly seen member of this group. Others include atypical teratoid/rhabdoid tumor and the so-called primitive neuroectodermal tumor. Histologically, they all share the presence of primitive appearing small round blue cells with high N:C ratio, hyperchromatic nuclei and brisk mitotic activity.