The development of diagnostic applications of ultrasound has tended to follow the evolution of electronic technology, and while this development has resulted in benefit to the patient, the relatively unsophisticated scientific ideas have been adequate. The present paper reviews briefly some elements of the development of diagnostic imaging in relation to their scientific bases. The discussion is particularly concerned with the popular grey-scale pulse-echo systems and the philosophy behind future developments that may lead to the successful pursuance of tissue differentiation, and the optimisation of instrumentation. Of necessity, the discussion touches the more fundamental areas of measurement of the ultrasonic parameters of tissues, the development of tissue models, the measurement of transducer field characteristics, the studies in visual perception; and is briefly linked to biological effects and therapy.