Fluctuations in heart rate (HR) and arterial blood pressure occurring at respiratory and lower frequencies have long been recognized. However, their significance remained obscure until the monitoring of fetal heart-rate variability (HRV) was appreciated. Recent studies suggest that HRV may reflect the sympatho-vagal interactions that modulate cardiovascular function. Analysis of HRV can provide a noninvasive measure of central autonomic outflow and autonomic reflex functions. A mathematical and signal-processing technique called power spectral analysis (PSA) has been used extensively to quantify HRV. Several frequencies of HR oscillations can be quantified: the low-frequency peak (LFP 0-0.04 Hz), the mid-frequency peak (MFP 0.05-0.15 Hz) and the high-frequency peak (HFP 0.15-0.4 Hz). In this review article, the physiological origins of these HR fluctuations are described and the changes of HRV by different pathophysiological states are also discussed. In conclusion, spectral analysis of HRV may provide important insights regarding the influence of anesthesia on cardiovascular neural control and anesthetic depth and the monitoring may be developed as a very useful non-invasive tool for modern anesthesia in the near future.