There are two major reasons for having this symposium. First, Tohoku Univer­ sity is the place where ultrasound investigations in Japan originated. Starting from the research studies of Professors H. Nukiyama and Y. Kikuchi of To­ hoku University, Professor J. Saneyoshi of Tokyo Institute of Technology, and Dr. K. Kato of Osaka University - all graduates of Tohoku University - the results spread to all parts of Japan. More recently we have had acoustic macro­ scopic studies by researchers like Professor N. Chubachi. As regards tissue char­ acterization, which was the main theme of the symposium, the collaboration among research workers in Japan and the United States started 10 years ago between Professor F. Dunn of the University of Illinois and staff members of Tohoku University and the Tokyo Institute of Technology. So this conference commemorates the lOth anniversary of that joint research effort. The second reason for this conference is that the application of ultrasound has become wide spread and indispensable in the routine clinical activities of medicine. But there have not been many breakthroughs in terms of quantitative and qualitative measurement of the living body tissues. Also, there are many problems with regard to practical application. There are various points that have not been elucidated yet as to the physical and acoustical characteristics of ultrasound itself. The methodology has not in all cases been well established. Therefore, the scientific elucidation of these areas is essential.