Tracer techniques have provided new insight in cardiology by allowing noninvasive studies of myocardial perfusion, function, metabolism, and, more recently, ligand-receptor interaction. Positron emission tomography allows accurate quantification and the use of natural substrates labelled with 11C, 13N, or 15O. Myocardial metabolism is complex and utilizes a number of substrates, primarily fatty acids. Fatty acids utilization can be studied with 11C palmitate, while 11C acetate more selectively traces TCA cycle activity and reflects myocardial oxygen utilization. Glucose uptake can be traced using 18F deoxyglucose, a glucose analog that is a substrate for hexokinase but is not further metabolized. Flow and oxidative glucose metabolism are usually coupled, and thereby the uptake of FDG and perfusion tracers are usually similar. In myocardial ischemia, however, glucose utilization can persist due to anaerobic glycolysis, and its uptake is frequently enhanced. Clinical applications of the use of metabolic studies in patients with ischemic heart disease are presented.