Heart rate variability (HRV) has been studied for years as a measure of physical well being. The amount of spontaneous variability in heart rate (HR) has been shown to decrease with various pathological states and to gradually improve during regression of the disease process. A mathematical and signal-processing technique called power spectral analysis (PSA) has been extensively employed to quantify HRV. When a continuous segment of HR data is analyzed by PSA, several frequencies of HR oscillations can be quantified. These are the low (0–0.04 Hz), mid (0.05–0.15 Hz) and high (0.15–0.4 Hz) frequencies. The physiological origins of these HR fluctuations are described and the changes in the frequencies of oscillation observed in various pathophysiological states are discussed. Several variations in processing techniques are reviewed. PSA of HRV could serve as a useful non-invasive marker of autonomic balance influencing HR, and may find an important role in patient monitoring before, during, and after anesthesia.