Teletherapy techniques include conventional external beam therapy, three-dimensional conformal therapy, intensity-modulated radiation therapy, stereotactic radiosurgery, and Gamma Knife radiosurgery. Brachytherapy is the process of placing a radioactive source next to or within a tumor. Ruthenium-106, palladium-103, and iodine-125 are commonly used isotopes for the treatment of ophthalmic tumors. Several treatment parameters, such as target volume, total dose, and fractionation, influence tissue effects. Larger doses per fraction (>200 cGy) are associated with a greater tendency for late side effects such as severe dry eye, cataract, radiation keratitis, and optic neuropathy. Normal tissues vary greatly in their tolerance to external beam radiation. With continued improvements in radiographic imaging techniques and a better understanding of radiation tissue effects at the molecular level, it will be possible to precisely focus therapeutic dose while minimizing damage to surrounding normal structures. In this chapter, we review the basic principles of radiation therapy and its application in the definitive, adjuvant, salvage, and palliative management of a variety of ophthalmic cancers.