The distinction of left ventricular (LV) dysfunction caused by fibrosis from that arising from viable (hibernating and/or stunned) myocardium has important implications for the management of patients with ischemic cardiomyopathy. In many of these patients, LV function can improve significantly after revascularization. Using positron emission tomography (PET), increased glucose uptake in dysfunctional segments with reduced blood flow at rest (PET mismatch) indicates presence of viable myocardium, whereas a concordant reduction in blood flow and glucose uptake (PET match) reflects myocardial scar. The average positive and negative predictive accuracies of PET for predicting improved function after revascularization are 76% and 82%, respectively. The greater the number of viable myocardial segments, the greater is the probability that revascularization will improve global LV function and, consequently, improve heart failure symptoms and survival. Early revascularization affords a more complete recovery of LV function. In patients with PET mismatch, early referral to revascularization improves long-term survival over medical therapy, regardless of symptoms. In patients without PET mismatch, long-term survival is similar with medical therapy or revascularization especially if minimal or no anginal symptoms are present. Noninvasive evaluation of myocardial viability should be an important component of the diagnostic evaluation of patients with heart failure due to coronary artery disease. This approach will likely enhance the selection of patients with poor cardiac function in whom revascularization will likely improve both the quality and quantity of life.