This volume contains the Proceedings of the Sixth International Symposium on Acoustical Holography and Imaging, held in San Diego, California, February 4-7, 1975. The title of this symposium differs from that of the first four by the addition of the word "Imaging", reflecting an increase in emphasis on nonholographic methods of acoustical visualization. For convenience, no change has been made in the title of this published series. The 38 papers presented here define the state-of-the-art in the rapidly developing field of acoustical holography and imaging. Many of them describe applications in such fields as medical diagnostics, microscopy, nondestructive testing, underwater viewing, and seismology. The Editor recognizes the diligent efforts of the authors in advancing the technology of Acoustical Imaging and thanks them for preparing and submitting descriptions of their work. The papers were selected with the able assistance of the Program Committee that con­ sisted of P.S. Green, Stanford Research Institute; J.F. Havlice, Stanford Microwave Laboratory; B.P. Hildebrand, Battelle Northwest; D.R. Holbrooke, Children's Hospital of San Francisco; P.N. Keating, Bendix Research Laboratories; A. Korpel, Zenith Radio Corporation; B.J. McKinley, Lawrence Livermore Laboratory; A.F. Metherell, University of Miami; J. Powers, Naval Postgraduate School; and F.L. Thurstone, Duke University. The Editor appreciates the help of the session chairmen: D.R. Holbrooke, Children's Hospital of San Francisco; Mahfuz Ahmed, Zenith Radio Corporation; R.K. Mueller, University of Minnesota; G. Wade, University of California at Santa Barbara; B.P.