In this paper, the principal applications of nuclear medicine to studies of the heart are described. First, gated cardiac blood pool imaging is discussed, then thallium-201 myocardial imaging, myocardial infarct scintigraphy with 99mTc pyrophosphate, and evaluation of intracardiac shunts. In gated cardiac blood pool imaging, the patient's red blood cells are labeled with 99mTc. Images of the cardiac blood pool are then obtained in multiple projections and displayed in an endless-loop cine display. Quantitative indices of cardiac function are readily obtained, and a variety of functional images can be generated. Blood pool imaging may also be performed with use of a first-pass technique that yields similar information. Applications of blood pool imaging are discussed. The theory and techniques of planar and tomographic thallium-201 myocardial imaging are described, together with their application in the diagnosis of coronary artery disease. The prognostic value of thallium imaging is also examined. Myocardial infarct imaging with 99mTc pyrophosphate is described, and clinical indications are reviewed. Left-to-right cardiac shunts can be evaluated by following the first transit of a bolus of radiopharmaceutical through the lungs. Right-to-left shunts may be evaluated by injection of 99mTc macroaggregated albumin.