Stereotactic radiosurgery is the accurate focusing of ionising radiation onto an intracranial target, and was pioneered in Sweden in the early 1950's. It has proven effective in the treatment of arteriovenous malformations, acoustic tumours and many other small brain lesions. In the last decade the commercial availability of a gamma ray system has led to other centres following the work in Stockholm; more recently, various techniques utilising linear accelerators, (LINACs), have been developed. Both types of system necessitated special dosimetry and treatment planning methods for the small, high dose-gradient circular fields. Localization of the lesions requires angiography, computerised tomography (CT) or magnetic resonance imaging (MRI).