The latest progress in acoustical holography and related research areas, generally involving imaging by means of acoustic waves, was discussed and treated in depth at the Fourth International Symposium on Acoustical Holography, held in Santa Barbara, California on April 10-12, 1972. This volume contains the proceedings of that symposium. As the papers presented here indicate, a num ber of startling advances have been realized in the state-of-the-art since publication of Volume 3 of Acoustical Holography. Progress has been partic ularly impressive in the field of acoustical imag ing. The Fourth International Symposium represents something of alandmark conference in this respect. The scope of this volume is substantially broad er than the term "acoustical holography' usually implies and encompasses the whole area of visualiza tion, detection, and recording of sound fields whether with long wavelengths, microwaves, or with extrememly short sound wavelengths. The 37 sympo sium papers appear here each as aseparate chapter. In general, the work reported deals mainly with ex perimental and theoretical developments in the a bove areas. This work has significant practical potential use in terms of seismic sensing, under water imaging, non-destructive testing, real-time acoustic microscopy, and medical diagnosis.