Radiation-induced optic neuropathy (RION) is a devastating late complication of radiotherapy to the anterior visual pathway resulting in acute, profound, irreversible visual loss. It is thought to be a result of radiation necrosis of the anterior visual pathway. Visual loss may be unilateral or bilateral; simultaneous or sequential. RION occurs commonly between 10-20 months, with an average of 18 months after treatment; but the onset may range from three months to 9 years. Cumulative doses of radiation that exceed 50 Gy or single doses to the anterior visual pathway or greater than 10 Gy are usually required for RION to develop. Several factors are associated with a higher risk for developing RION or for RION occurring with lower total doses of radiation. These include age, pre-existing compression of the optic nerve and chiasm by tumour, concurrent chemotherapy or previous external beam radiation. MRI, the investigation of choice for identifying radiation injury to the visual pathway, may show abnormalities before the loss of vision. Typically, the unenhanced T1- and T2-weighted images show no abnormality, but the optic nerve will show enhancement on T1-weighted images with MRI. Treatment with systemic corticosteroids, anticoagulation and hyperbaric oxygen has been generally unsuccessful and disappointing. If visual dysfunction is detected early, hyperbaric oxygen might be beneficial if treatment is initiated within 72 hours of visual loss. Because of the poor prognosis associated with RION, the risk of its potential development should be factored into the decision to irradiate the brain.