This chapter discusses the predominant genetic abnormalities associated with the common types of brain tumors. Astrocytomas tumors of one form of glial cell, can occur in all areas of the brain and spinal cord in children and adults. Ependymomas are a category of glioma derived from the ventricular lining that can occur at many locations within the brain and spinal cord of adults and children predominantly associated with the ventricular system. Oligodendrogliomas are a type of glioma which occur mainly in the cerebral hemispheres of adults and are derived from the oligodendrocyte cell lineage. Medulloblastomas, the most common malignant primary brain tumors of childhood, are small cell neoplasms which arise in the cerebellum. Rhabdoid tumors occur in children and usually present as tumors in kidney or brain although capable of occurring in all locations of the body. In the brain these tumors may consist purely of rhabdoid cells or contain areas of rhabdoid cells adjacent to mesenchymal and epithelial tissue.