Exciting new developments and discoveries of the last two decades are beginning to shed light on the complex biology of brain tumors and are advancing our understa- ing of the cellular and molecular processes involved in their initiation, progression, and clinical and biological behavior. The disease process in brain tumors is quite complex and the resulting tumors are characterized by a high degree of biological and clinical diversity. Thus, despite the advances of the last two decades, prognosis for patients with malignant brain tumors remains abysmal. Significant progress in the diagnosis, treatment and, ultimately, prevention of these tumors will require both the timely h- nessing of the advances in basic and clinical brain tumor research, and a continuing concerted effort at increasing our understanding of brain tumor biology, in particular, the molecular genetic changes and perturbations of cellular pathways involved in brain oncogenesis and which drive the biological and clinical behavior of the tumors. Brain tumor diagnosis and prognosis, which is still largely based on histopathology and other clinical criteria, will, in the future, acquire a significant molecular component, with the incorporation of knowledge of genes that are mutated, over-expressed, deleted, silenced, or functionally altered in the tumors. Treatment strategies for brain tumors, rather than being empirical, will be rationally developed based on an understanding of the cellular and molecular mechanisms and targets that have been activated, suppressed, or otherwise altered.