Ultrasound is now a widely used technique in medicine. The precise mechanisms by which ultrasound interacts with tissues are still not fully understood, however, and it is possible that ultrasound presents a small hazard to those who receive it. An understanding of the possible mechanisms by which ultrasound may be hazardous is necessary if reliable safety levels are to be set. One possible damage mechanism is transient cavitation, which is the creation, expansion and collapse of small bubbles in tissues in response to the variations in pressure produced by the ultrasound wave. During the collapse stage, very high temperatures can be produced within the bubble and it is likely that free radicals are formed. Sonoluminescence is the name given to the light emissions that accompany transient cavitation, and is an indication that cavitation has occurred. In this thesis the phenomenon of sonoluminescence, and various factors that influence it, are investigated under similar conditions to those obtaining in clinical practice. When the effect of physiotherapeutic ultrasound on a tank of water was investigated and light output was detected using either a photomultiplier or an image intensifier, sonoluminescence was found to increase with increasing ultrasonic intensity above a well defined threshold. Sonoluminescence also increased with increasing temperature and was found to depend on duty cycle and standing wave ratio. Subsequently sonoluminescence was also recorded from water after insonation with quite short pulses of ultrasound, but thresholds, were much higher than with long pulses. The effect of ultrasound on monolayers of cells growing in culture was found to depend on the position of the monolayer in the standing wave field. Finally a direct attempt to measure sonoluminescence from the human cheek was made, but none was observed.