The noninvasive assessment of myocardial viability has proved clinically useful for distinguishing hibernating myocardium from irreversibly injured myocardium in patients with chronic ischemic heart disease or recent myocardial infarction who exhibit marked regional and global left ventricular dysfunction. Noninvasive techniques utilized for detection of viability in asynergic myocardial regions include single-photon-emission CT perfusion imaging with 201Tl or one of the new 99mTc-labeled perfusion agents, positron emission tomographic imaging of perfusion and glucose uptake, low-dose dobutamine echocardiography for assessment of inotropic reserve, and contrast echocardiography for evaluation of microvascular integrity. The greater the number of viable myocardial segments by any of these techniques, the greater is the probability of improvement in regional and global left ventricular function, improvement in heart failure symptoms and functional capacity, and enhanced survival after revascularization. Patients with a decreased left ventricular ejection fraction and extensive myocardial viability treated medically have a high cardiac event rate. Similarly, patients with poor viability preoperatively who still undergo coronary bypass surgery have a high rate of early and late cardiac death or need for transplantation compared with patients with greater viability. Finally, some patients with severe ischemic cardiomyopathy referred for cardiac transplantation may have substantial zones of hibernation and may still be candidates for coronary bypass surgery, even in the absence of angina.