We are interested in the interaction of high-energy ultrasound with biological tissue with regard to cancer therapy. The physical and biological aspects of this are reviewed and summarized in this paper. The preliminary results give reason to hope, as ultrasound is easy to focus [correction of focuse] and might be applicable as a supplementary therapeutic technique in ultrasound-accessible tumors. The changes observed in biological studies are clearly a function of the physical parameters. Therefore, it is possible to calculate and predict the biological effects. Recent results have shown the potential of magnetic resonance imaging in monitoring ultrasound therapy online. The capability of MRI to measure temperature will be an important tool for the safe and reliable application of this new therapeutic approach. The role of ultrasound therapy in the management of human tumors must be clarified in carefully conducted clinical trials.