The impact of CNS tumors during childhood and adolescence has been steadily increasing. In many countries, brain and spinal cord tumors are now second in frequency only to leukemia as a cancer affecting children, and the most common cause of cancer mortality in the young. In the United States, brain tumors are now more common than acute lymphoblastic leukemia, and the proportion of cancer deaths due to CNS tumors has nearly doubled during the past 25 years. Worldwide, approximately 30,000-40,000 children develop CNS tumors each year, and the majority do not survive. Compared with most other malignancies that occur during childhood, CNS tumors have not been treated with comparable success in treatment outcome. Also, no specific risk factor, or set of risk factors, has been identified to explain a substantial proportion of CNS tumor occurrence. In many countries, CNS tumors are now the greatest challenge in pediatric oncology.