The use of ultrasound in medical diagnosis has been known and experimented with for some 20 years. It is only in the last few years that its application has become fairly widespread, primarily because of the high correlation factors obtained by investigators in the neurological field in the Scandinavian countries using stykographic (A-scan) techniques. Now, more-sophisticated methods, such as ultrasonic tomography, are beginning to appear in the neurological area. While some equipment is available that is capable of localizing many intracranial masses, work remains to be done in the interpretation of the resulting tomograms. Because physicians, on the one hand, are generally unfamiliar with the properties of ultrasonic waves and their interaction with matter and physicists, on the other hand, are not well-acquainted with human anatomy, physiology, and pathology, close cooperation is needed between the disciplines of acoustics and medicine to ensure successful advancement of medical ultrasonography. A discussion of some of the biological and physical factors involved and a report of recent progress is given. The neurological area is chosen for discussion because almost all possible factors of importance in interpretation are found here. The conclusions apply to other body areas.