This paper reviews research on the physics of liquid impact and the behaviour of cavities when a shock passes over them. It is shown that the problems are related since when a cavity collapses near a solid surface, or is collapsed by a shock wave, the collapse is asymmetric and a liquid jet is produced which can impact an adjacent solid. The implications of this research to shock wave lithotripsy are emphasized. Finally, the paper describes the development of a device, based on liquid jet impact, for fragmenting kidney stones.