The journal's focus this month on coronary physiology is very timely. Identification of patients suffering from angina who have no significant epicardial artery obstruction presents a diagnostic and therapeutic dilemma. There is increasing interest in the various symptomatic presentations that sound like myocardial ischemic symptoms but with no obstructions. There is also continuing interest in identifying the angiographic intermediate lesions in the major arteries that may be capable of producing ischemia. Technologies miniaturized to the size of guidewires, including pressure transducers, Doppler transducers, and temperature thermistors, have enabled invasive measurements of coronary physiology such as fractional flow reserve (FFR), instantaneous wave free ratio (iFR), coronary flow reserve (CFR), and index of myocardial resistance (IMR), as well as variations on those methods. Noninvasive methods for evaluating coronary physiology have evolved and can reflect some of these parameters as well. Absolute flow and quantification of flow in specific regions of the myocardium is the purview of positron emission tomography (PET).