An extensive review is presented and 99 references cited relating to clinical radioisotope scanning for the detection of tumors and mapping of internal organs by determining the spatial distribution of gamma -ray- or positron-emitting radioactive nuclides localized in or surrounding the object under study. It is pointed out that successful scanning does not require administration of the high concentration of test materials which are necessary to produce opacity to x rays in certain diagnostic roentgenographic procedures. The important criterion is the relative concentration of the isotope in the organ or tumor with respect to its surroundings, rather than the absolute concentration. Radioisotope scanning has recently achieved greater importance in medical diagnosis largely due to: development of improved radiation detection equipment, development and production of radiopharmaceutical agents that concentrate in particular organs or tissues, and better understanding of the factors necessary to obtain scanning images that are readily interpretable by the clinician. Improvements in scanning equipment are discussed with regard to: the detector and its collimator, apparatus to move the detector, and display of the readings so that the information can be interpreted. Development of instrumentation to meet various requirements for diagnostic tests is discussed, and specific uses of individual radioisotopesmore » in various techniques are outlined as they are applied to organ systems (bone, brain, kidney, liver, heart, pancreas, spleen, thyroid). Detection of tumors in various organs by demonstrating the localization of radioactivity in a given area is also considered, but, it is concluded, the presence of such activity is not specific for tumor tissue, either benign or malignant. In the liver, kidney, spleen, and thyroid any carcinomatous involvement almost always is visualized as a hole in, or displacement of the normal functioning tissue. At present, no scanning procedure is available which will detect malignant tumors in any part of the body by means of selective isotopic localization in malignant tissue. However, three main lines of investigation are mentioned which indicate such a possibility for the future: tumor uptake of labeled porphyrins, tumor uptake of labeled antihuman antibodies, and incorporation of radioactive halogenated analogs of thymidine into the DNA of neoplastic cells. (BBB)« less