Single photon emission tomography (SPET) is making its impact on the practice of clinical nuclear medicine. Special purpose and general purpose instrumentation has become available with significantly improved performance capabilities. The advantages to be expected from this new approach to noninvasive medical imaging methodology include improvement of contrast resolution of the image and the ability to quantify the information in the three-dimensional section scan. It has been shown that SPET can replace planar imaging of the brain and that it significantly improves lesion detection in the liver. Organ volume measurements can be achieved and tracer uptake parameters calculated. This field is still at its beginning. Instrumentation will change and significant improvements in reconstruction algorithms will extend the clinical usefulness of these techniques. Even if absolute measurements may not be achievable in the body, it is possible to distinguish an abnormal population from a normal one. Sequential data may then be recorded and integrated into serial treatment protocols. Progress in the labeling of clinically useful tracers with suitable radionuclides [123I, 99mTc, 197Rt, 81mKr, 111In, etc.) is needed in order to achieve further significant progress in the areas of clinical applications.