In the United States, sudden cardiac death is a major public health problem, accounting for approximately 300,000 deaths annually. Accurate identification of those patients at highest risk for this event has been problematic. The use of signal-averaged electrocardiography, Holter monitoring and assessment of left ventricular function have been shown to be predictive of future arrhythmic events in patients after a myocardial infarction. However, the clinical utility of these tests has been limited by their low sensitivity and positive predictive value. It has become increasingly clear that the autonomic nervous system is extremely important in the pathogenesis of ventricular arrhythmias and sudden cardiac death. The two most important techniques used to study the autonomic nervous system--heart rate variability and baroreflex sensitivity--are reviewed, and the clinical and experimental data suggesting that these techniques are powerful predictors of future arrhythmic events are discussed in depth.