Radiotherapy (RT) is a proven curative and palliative therapeutic tool in the treatment of a wide variety of primary and metastatic brain tumors in adults. Recent advances in multimodality therapy have led to improvement in survival for many cancer patients. As survival has improved, more attention has been directed toward long-term treatment-related morbidity. Specifically, the effect of RT on the long-term cognitive performance of these patients is a major concern. This article reviews the neurocognitive effects of cranial RT on adult patients with brain tumors. Analyses of neurocognitive function are confounded by factors such as surgery, chemotherapy, tumor characteristics, tumor progression, concurrent medical illnesses, neurologic comorbidity, and medications that can contribute to neurocognitive deficits. Risk of deficits after cranial RT is associated with high RT dose, large fraction size, larger field size, and extremes of age at time of treatment. Using modern techniques with moderate total doses (50 to 54 Gy), conformal RT, conventional fractionation, and advanced planning imaging and software, the risks of neurocognitive deficits are quite small and greatly overshadowed by deficits caused by the tumor itself. Further studies need to be undertaken to elucidate the degree and cause of cognitive decline in adult patients undergoing multimodality therapy for cranial tumors.