OBJECTIVE To describe heart rate variability (HRV) analysis, especially time- and frequency-domain analyses, and some of its investigational applications in clinical cardiovascular medicine. METHODS We provide a brief introduction to the magnitude of sudden cardiac death and the factors that influence life-threatening ventricular arrhythmias as a backdrop to the potential importance of the autonomic nervous system and how this system might be assessed by the analysis of HRV. METHODS We reviewed the literature from 1973 to 1994 that described beat-to-beat changes in heart rate, heart rate signal recording and processing, and investigational applications of HRV analysis to cardiovascular medicine. RESULTS Beat-to-beat changes in heart rate are partly influenced by the autonomic nervous system. Briefly, changes in sympathetic input to the sinoatrial node affect low-frequency HRV, whereas changes in parasympathetic input affect high-frequency HRV. Multiple physiologic and nonphysiologic determinants of HRV exist, and therefore analysis of HRV as a direct "window" to autonomic tone is problematic. CONCLUSIONS In selected patient populations, analysis of HRV yields important information about sinoatrial responsiveness to autonomic input and mortality risk stratification. Routine application of HRV analysis to clinical cardiovascular medicine awaits further investigation, however.