Heritable syndromes and ionizing radiation, the only two established causes of primary CNS tumors, each account for only a few percent of cases of this disease. Findings are inconclusive for other suggested risk factors, including head trauma, prior infections, and pesticides, among others. The apparent secular increase in brain tumor rates is most likely predominantly attributable to improvements in diagnosis. The differing patterns of occurrence of gliomas and meningiomas suggest the need for separate study. Future studies should aim for more precise assessment of environmental exposure and simultaneous consideration of individual susceptibility.