The paper reviews the use, design and performance of the various forms of radio-isotope instrumentation, with some comment on the historical development of the subject, and possible developments, in the future. Earlier surveys published by the IEE have described nuclear measuring instruments for use in the laboratory and in nuclear-reactor experiments and other installations. Here it is proposed to review radio-isotope instrumentation, i.e. an instrument comprising a radio-isotope source and some measuring component, as used for investigational work in science and engineering, but more particularly in industry to improve both productivity and the quality of the product being manufactured. Emphasis is given throughout to the special techniques involved, and attention is also given, where possible, to applications in electrical engineering. An attempt is made to classify the various applications discussed and to give examples of each class.