||Diagnostic ultrasound is a rapidly developing imaging technology widely used in both industrialized and developing countries. For certain diagnostic applications, ultrasound has replaced commonly used radiographic imaging techniques as the method of choice, and it has also made possible new areas of diagnostic investigation. Moreover, equipment for ultrasound imaging tends to be cheaper and more widely available than imaging equipment requiring the use of ionizing radiation. This combination of factors has resulted in the proliferation of diagnostic ultrasound units, and in some cases their use by individuals without proper training, or under conditions of inadequate control. This report, the outcome of a recent WHO Study Group, is concerned with the essentials, principles, and standards of training for this important technology. The Study Group has analysed problems in the effective use of diagnostic ultrasound and reviewed current training practice worldwide. For the first time, outlines of recommended training curricula for the general, advanced, and specialized use of diagnostic ultrasound are presented. Recommended standards for training programmes, training centres, and the training process are also discussed. The report highlights the role played by professional societies, but also calls attention to the need for appropriate legislation and regulation. The Study Group's recommendations are relevant to all those involved in the use of diagnostic ultrasound technology, even in countries where existing standards of practice are high. In addition to ultrasound specialists, the report should be of particular interest to those responsible for medical education or for formulating policies regarding the use of health technology.