This volume represents the proceedings of the 21 st International Symposium on Acoustical Imaging, which was held at the Surf and Sand Hotel in Laguna Beach, California, March 28-30, 1994. These unique and highly interdisciplinary series of symposiums have met at intervals of roughly 18 months over the past 30 some years. In general these meetings are devoted to all aspects and all fields of imaging that use acoustics. The meetings are usually small, with 100 to 200 participants, and stimulate useful interchanges across disciplines. These are the only regular meetings where the major researchers in all areas of acoustical imaging can come together to interchange ideas and new concepts. The Acoustical Imaging Symposiums have long been regarded as the premier meeting of this type in the general field of acoustics. The highly regarded and carefully edited proceedings have been published regularly by Plenum Press. I am proud and honored to serve as editor of the 21st volume in this series. The 21st Symposium was attended by well over 100 participants from some 18 countries. During the three day symposium, 94 scientific presentations were given, 66 as formal lectures and 28 in a poster format. Sufficient time was available during the conference, both following the presentations and informally during meals and breaks, for active discussions among all participants. Over 80 of the presentations have been selected for inclusion in these proceedings.