Sudden cardiac death remains one of the leading causes of death in western societies. Accordingly, the ability to identify patients at high risk of sudden cardiac death is important so that appropriate treatments can be used efficiently. Recently, T wave alternans (TWA) has emerged as a promising new test for such risk stratification. TWA is a heart rate-dependent measure of arrhythmia vulnerability, with maximal predictive accuracy at sustained, regular heart rates of 100 to 120 bpm. In the clinical setting, these conditions may be achieved by either exercise or atrial pacing. TWA has been shown to predict inducibility of ventricular tachycardia with programmed stimulation and also spontaneous arrhythmic events. TWA has been successfully applied to diverse populations, including patients with coronary artery disease, nonischemic cardiomyopathy, congestive heart failure, and implantable defibrillators. Despite these encouraging results, the role of TWA to guide clinical therapy still needs to be elucidated better.