Heart rate variability (HRV) is a measure of autonomic nervous system balance/imbalance. Measurement and analysis of HRV can be classified into time domain analysis and frequency domain analysis. lime domain analysis is a general measure of autonomic nervous system balance and is based on altered versions of the measurement of the standard deviation of heart period, defined in terms of the sinus R-R intervals over time. There are six commonly used time domain measurements: SDANN, 24-hr SD, SD, RMSSD, RR50, and% RR50. Frequency domain analysis uses either autoregressive orfast Fourier transform techniques to delineate parasympathetic (high-frequency components) from sympathetic (low-frequency components) of autonomic control. This study explores the clinical applications of HRV with respect to respiratory sinus arrhythmia, exercise, mortality, sudden cardiac arrest, myocardial infarction, and congestive heart failure.