Primary brain tumors, including gliomas and medulloblastomas, often represent the most devastating and difficult-to-treat tumors, and are thought to arise from glial cells and/or their precursors or the external granule cell layer, respectively. The majority of genetic alterations characteristic of the human brain tumors are thought to occur in genes encoding proteins involved in signal transduction or cell cycle regulation. Accurate recapitulation of these genetic alterations using genetically engineered mouse models allows for in vivo modeling of brain tumors with similar histopathology, etiology, and biology. These mouse models, in turn, increase our understanding of brain tumor initiation, formation, progression, and metastasis, providing an experimental system to discover novel therapeutic targets and test various therapeutic agents.