Methods, instrumentation, performance data, clinical use and results of SPECT are presented. Since SPECT is yielding functional tomograms, morphological data are not the major concern with this method. Essentially 99mTc labeled radiopharmaceuticals, 201Tl chloride or 133Xe gas are in current use. SPECT may be considered as a supplement to ultrasound in detecting space-occupying lesions of the liver (combined accuracy 95%). Cranial SPECT already allows quantification of regional cerebral blood flow following inhalation of 133Xe gas. The sensitivity of myocardial SPECT with 201Tl (three-dimensional reconstruction) at rest in patients with severe coronary artery disease was higher than 90%. To further develop SPECT as a functionally oriented supplementary method to other radiodiagnostic procedures, further improvements are required as to performance, reconstruction algorithms and radiopharmaceuticals.