An invasive, high-grade (WHO grade IV) embryonal tumor defined both by histologic grade and location in the cerebellum. Even though this tumor shares histologic features with other central nervous system (CNS) primitive neuroectodermal tumors, it has been historically regarded as a distinct entity. Recent molecular studies including genomic and gene expression profiling, as well as signaling pathway dysregulation and biologic studies, have now justified its historic clinical definition as a distinct clinicopathologic entity with predominant occurrence in children. Most are sporadic. Less frequent occurrence in the setting of hereditary syndromes including Turcot’s syndrome with germline mutation of the adenomatous polyposis coli (APC) gene and Gorlin’s syndrome, the nevoid basal cell carcinoma syndrome with germ line mutation of the PTCH gene is seen in less than 2% of medulloblastomas.