Autonomic nervous system (ANS) consists of sympathetic and parasympathetic components. ANS has a role in heart function and contraction and relaxation of smooth muscle in various internal organs. Several tests were introduced to assess ANS function. Heart rate variability (HRV) is the most common method to evaluate ANS function. HRV is defined as a physiologic variation between sinus beats, which is calculated into standard deviation of N-N interval. The indices derived from beat-to-beat variation are calculated based on time-domain analysis. Fourier transformation visualizes frequency-based graphs from the time-domain tachograms. Areas in the low frequency represent sympathetic predominant function, while high frequency areas show parasympathetic tone. Despite absence of the reference ranges of the HRV indices, studies have shown the ranges of the indices in their populations. Decrease in HRV may be a poor prognostic factor for heart disease, infection, and neuropsychiatric disorders. It is reported that HRV biofeedback improved various disorders related to ANS function. In addition, the results of HRV tests may guide functional approaches. In conclusion, the awareness on ANS function and HRV may help to provide patients with risk stratification and proper management.