One of the first applications of ultrasound was in submarine sonar equip­ ment. Since then ultrasound has found increasing applications, particularly in industry, but increasingly in biomedicine. For many years ultrasound has been used in physical therapy, although only in the past decade or two has it evolved from laboratory curiosity to a well-established diagnostic imaging modality. Ultrasound is now a widely accepted, indeed pervasive, diagnos­ tic and therapeutic tool in the medical field, and its applications are increasing rapidly. Our intent in developing this book is to provide a coherent tutorial intro­ duction to the field of medical ultrasound at a level suitable for those en­ tering the area from either medical or scientific backgrounds. The topics discussed should be of interest to nearly all medical and health care per­ sonnel needing to understand or operate ultrasonic devices, including clini­ cians, medical technicians, physiotherapists, medical physicists, and other biomedical scientists interested in the field. The book opens with a description of the basic principles of propagating acoustic waves, explains how they interact with a wide range of biological systems, and outlines the effects they produce. To provide practical infor­ mation to operators of ultrasound equipment, we have included thorough coverage of the details of ultrasonic instrumentation and measurement techniques, and set forth the framework for an effective quality assurance program.