The application of ultrasonics has been one of the major developments in ophthalmology in the last two decades. Diagnostic ultrasound of the eye and orbit, together with ocular biometry (especially in the field of lens implantation), are now routine investigations, while ultrasonic surgery of the lens (phacoemulsification) has become a well-established part of our surgical armamentarium. The historical development of diagnostic ultrasound is reviewed, its physical basis is recalled, present-day techniques and instrumentation are outlined, and future possibilities considered.