Heart rate variability (HRV) is a convenient and non-invasive method for evaluation of autonomic nervous system. The measurement and analysis of HRV can be done by time domain analysis or frequency domain analysis. Time domain analysis is based on altered versions of the measurement of the standard deviation of the heart period. Frequency domain analysis transforms the RR interval to a power spectral analysis of heart rate variability. Studies of heart rate variability were used not only in the field of cardiovascular disease, but also for analysis of noncardiovascular disease and healthy individuals. We reviewed here the issues concerning heart rate variability, its measurement and clinical applications.