Orbital radiotherapy is a well-established method of treatment for severe Graves' ophthalmopathy, because of its anti-inflammatory and locally immunosuppressive effects. It has been used for 60 years. Conventional external x-ray and cobalt therapy have been abandoned, and most groups now use supervoltage linear accelerators (4-6 MeV). Cumulative doses may vary, but in most studies a cumulative dose of 20 Gy delivered over 2 weeks was utilized. Successful outcome depends on the selection of patients, because recent onset, active ophthalmopathy is much more favorably affected than longstanding, inactive disease. Inflammatory signs, recent onset eye muscle dysfunction, and optic neuropathy respond well to orbital radiotherapy, while proptosis and longstanding eye muscle restriction respond poorly. Overall, favorable responses have been reported, with few exceptions, in approximately 60% of cases. Combination of irradiation with high-dose systemic glucocorticoids provides better results than either treatment alone. Orbital radiotherapy is well tolerated and safe. Preexisting retinopathy (e.g., in patients with diabetes) is a contraindication to this treatment for the risk of further retinal damage. No case of radiation-induced tumors has so far been described after orbital radiotherapy for Graves' ophthalmopathy.