Glioblastoma multiforme (GBM) is a highly lethal brain tumor for which little treatment is available. The epidermal growth factor receptor (EGFR) signaling pathway is thought to play a crucial role in GBM pathogenesis, initiating the early stages of tumor development, sustaining tumor growth, promoting infiltration, and mediating resistance to therapy. The importance of this pathway is highlighted in the fact that EGFR is mutationally activated in over 50% of GBM tumors. Consistent with this, we show here that concomitant activation of wild-type and/or mutant (vIII) EGFR and ablation of Ink4A/Arf and PTEN tumor suppressor gene function in the adult mouse central nervous system generates a fully penetrant, rapid-onset high-grade malignant glioma phenotype with prominent pathological and molecular resemblance to GBM in humans. Studies of the activation of signaling events in these GBM tumor cells revealed notable differences between wild-type and vIII EGFR-expressing cells. We show that wild-type EGF receptor signals through its canonical pathways, whereas tumors arising from expression of mutant EGFR^vIII do not use these same pathways. Our findings provide critical insights into the role of mutant EGFR signaling function in GBM tumor biology and set the stage for testing of targeted therapeutic agents in the preclinical models described herein.