Heart rate variability (HRV) is a measure of the beat-to-beat variations in heart rate. It is usually calculated by analyzing a time series of beat-to-beat intervals from the ECG. Various measures of heart rate variability have been proposed, which can be subdivided into time domain, frequency domain and non-linear measures. HRV is regarded as an indicator of the activity of autonomic regulation of circulatory function. HRV has considerable potential to assess the role of autonomic nervous system fluctuations in normal healthy individuals, in patients with various cardiovascular and noncardiovascular disorders and, in patients undergoing general anesthesia. HRV studies should enhance our understanding of physiological phenomena, perioperative cardiovascular changes, the actions of medications, and disease mechanisms.