The early attempts to use radiant energy in the treatment of eye disease form a dark and calamitous chapter in the history of radiotherapy. The unhappy consequences of such therapy, aided by a great deal of primitive and unsound experimental work, served to retard for a generation its development and applications to ophthalmology.Today, however, the radiotherapist approaching the special problems of ocular disease is sustained by the tremendous advance in the knowledge of the physical nature and properties of the agents used and by their dramatic increase in variety and number. These remarkable advances in the field of physics would be of little value had no reliable knowledge of the effects of radiation on the eye under clinical and experimental conditions also been simultaneously acquired. Such knowledge, withal incomplete and occasionally uncertain, permits the newer physical agents to be used without the risks of repeating the technical errors and encountering the ocular disasters which hitherto have helped surround radiotherapy with an impenetrable barrier of fear and distrust.In the past 12 years at the Royal Marsden Hospital, chiefly as a result of its fortunate association with Moorfields Eye Hospital, there has been excellent opportunity to study the applications of radiotherapy to ophthalmology. In the first half of this period the oldest, the sturdiest, and the most trusty of all isotopes, namely radium, was alone available and exclusively used. As the newer artificially produced radioactive substances became available, so certain of them have been chosen for use in eye work.