Due to the ever increasing interest in the use of non-invasive ultrasonic methods in medical diagnostics on the one hand and the specific effects of ultrasound in medical therapy on the other, the questions of safety and optimal applications are topical and of great importance. For this reason the symposium "Ultrasound Interaction in Biology and Medicine," initiated and supported by the "International Organization of Medical Physics," took place. The organizers were the Institute of Applied Biophysics of the Martin-Luther-University, Halle (Saale), German Democratic Republic, in association with the Society for Physical and Mathematical Biology of the GDR along with other scientific organizations. Renowned and internationally noted specialists in the field of ultrasonics reported on the latest findings regarding the biological interaction of ultrasound, which promised future improvements in the methods of ultrasonic diagnostics and gave and up-to-date insight into the biological effectiveness of ultrasound. We are pleased to be able to publish selected contributions to this symposium collected in one volume. The methods of investigation, theoretical considerations and results concerning the interaction of ultrasound on molecular, cellular and system levels contained herein will remain up-to-date for a long time to come, providing thought provoking material for further inter-disciplinary basic research and medical application.