High intensity beams of ultrasound may be focused at depth within the body, thereby producing selective damage within the focal volume, with no harm to overlying or surrounding tissues. The technique is thus noninvasive, insofar as the source of ultrasound energy is situated outside the body. The mechanism for cell killing is predominantly thermal, although acoustic cavitation may also occur. Ultrasound focal surgery was first conceived in the 1940s as a possible tool for creating selective damage in the brain for neurosurgical research; its potential for more widespread clinical use was not exploited at that time, probably because of the lack of facilities for providing precise visualisation and localisation of the damage. The availability of modern imaging techniques has encouraged a revival of clinical interest, and applications in ophthalmology, urology and oncology are currently being developed.