The 105 theses contained in this book are selected from those whose authors were present at the 20th International Symposium on Acoustical Imaging. held at Southeast University. Nanjing. China. during September 12-14. 1992. It was the first time that the symposium had been held in China. Our efforts to host the conference goes back to the 15th International Symposium on Acoustical Imaging held in Halifax. Canada. in 1986. We are glad that the 20th symposium has been successfully held at last. We are ardent for the symposium not only because we attach much importance to the field of acoustical imaging. but also because we admire the tradition of the serious academic exploration and friendly cooperation of the scholars attending the symposium. The theses in this book are from 21 countries and those by Mr. G. Wade. Takuso Sato. J. F. Greenleaf. K. J. Langenberg. and Wencai Yang are the specially invited papers. These theses cover such important fields of acoustical imaging as follows: 1. Mathematics and physics of acoustical imaging; 2. Components and industry application; 3. Applications in medicine and biology; 4. Applications in nondestructive testing; 5. Applications in geophysics; 6. Underwater acoustical imaging. All these theses reflect the latest progress in theory and technology. We are very grateful to all the authors who have provided these theses.