The formation of images by ultrasound is a fascinating study, with well-established, yet rapidly growing, applic­ ations in medicine and with increasing relevance to a surprisingly disparate set of problems in the non-destructive examination of materials and components. The present volume is a record of the research presented at the Twelfth International Symposium on Acoustic Imaging, held in London during July 1982. Whilst, therefore, it offers primarily a snap-shot in time of a rapidly developing field, it is so organized that it will also serve as a high-speed entry into the literature for someone embarking, for the first time, on· researches in this branch of applied science. As in previous volumes, some of the work reported is concerned with topics which, whilst of critical importance to the performance of any imaging system, - e.g. transducers, signal processing may not address themselves to image formation per se. A new departure is the inclusion of photo-acoustic imaging a subject of rapidly growing importance for many of the same application areas relevant to acoustical imaging.