Positron emission tomography (PET) permits the in vivo application of tracer kinetic principles as a tool widely employed throughout the biological sciences. With this ability, the noninvasive measurement of regional functional processes in the human heart has now become possible. Tracer amounts of compounds labeled with positron emitting isotopes of elements such as oxygen, carbon, nitrogen and fluorine as major constituents of living matter are administered intravenously or by inhalation. Given the high temporal resolution of modem PET scanners, the arterial tracer input function and the myocardial tissue response to it can be quantified noninvasively from serially acquired images. The high spatial resolution capability permits to derive from these images the true tracer activity concentrations in tissue and their time dependent changes. For example, near simultaneous measurements of regional myocardial blood flow, oxygen consumption (MVO2) and substrate utilization offer an opportunity to quantify the relationship between substrate delivery, transmembranous transport processes and intracellular substrate utilization. Furthermore, as modem PET scanners interrogate the entire left ventricular (LV) myocardium, the spatial distribution and heterogeneity of these processes in the normal and diseased myocardium can be mapped and evaluated.