Identifying preserved myocardial viability in the presence of severe regional left ventricular dysfunction is becoming increasingly more important for clinical decision-making to better select those patients with coronary artery disease who will benefit most from revascularization. 201Tl remains the most commonly employed radionuclide for detecting both ischemia and viability. A post-exercise defect showing complete or partial redistribution on delayed images implies transient ischemia and preserved viability with a 90% chance of exhibiting improved 201Tl uptake with repeat testing after coronary revascularization. Mild persistent defects with < 50% 201Tl uptake on 4-hour redistribution images also imply viability with a 60-70% probability of showing improved 201Tl uptake after repeat imaging following revascularization with concomitant enhancement of regional systolic function. In contrast, a severe persistent defect with < 50% 201Tl uptake compared to peak uptake has only a 15% chance of exhibiting improved perfusion and corresponding improved function after revascularization. Detection of defect reversibility on 201Tl imaging is enhanced by "reinjection" of a second 201Tl dose after acquisition of redistribution images. Initial and 4-hour rest/redistribution imaging has proven most useful for detection of viability in the resting state in patients with ischemic cardiomyopathy. The greater the extent of preoperative viability, the greater is the improvement in regional and global function after revascularization. 99mTc sestamibi has also been demonstrated to be extracted by myocardial cells in proportion to regional blood flow in the presence of viable myocites. Although this agent does not redistribute after intravenous injection, its > 50% uptake of the tracer implies viability and predicts improved regional function after revascularization. Finally, positron emission tomography with 18F fluorodeoxyglucose (FDG) is perhaps the most sensitive noninvasive imaging technique for detection of viability in stunned or hibernating myocardium. A mismatch pattern between regional flow and FDG uptake has approximately an 80-85% positive predicted value for predicting improved function in asynergic myocardial regions after revascularization. A match pattern where flow and FDG uptake are both reduced has an 80% negative predicted value for lack of functional recovery after revascularization.