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The paper aims to introduce the reader to the wide array of diagnostic devices, based on physical processes, which are now available or under development, and also to indicate aspects which require further technical development or even a new approach to design and construction. Recent technological progress has led to the availability in developed countries of many diagnostic techniques, e.g. X-ray and NMR imaging, positron emission imaging, thermography. The technical basis of a number of these methods is described with emphasis on the nature of the information which may be obtained. Some of these techniques do not involve any ionising radiation dose. This condition offers the possibility of repeated investigations of the same patient in order to follow the progress of disease and of treatment. It also suggests new possibilities for mass screening in order to detect people at risk. In some cases, engineering development has taken place in advance of scientific understanding of the response of biological tissue to the physical probe, and this has delayed full realisation of the potential of the method. Many of the newest devices involve high capital and running costs, and require well-qualified technical support. The need for low cost, robust devices is discussed with particular reference to the UK Health Service and the needs of developing countries.