Heart rate variability (HRV) has become a popular method to study autonomic modulation of the cardiovascular system. Cardiovascular health depends on the proper functioning of several physiological systems, including the autonomic nervous system, which exerts its function via a complex interaction between its two branches: sympathetic and parasympathetic. Additionally, heart rate variability has been widely applied in basic and clinical research studies for describing the extent of sinus arrhythmia, and for assessing the function of cardiac autonomic regulation quantitatively. In other words, it is a mirror of the balance of sympathetic and vagal activity. This book discusses the prognostic significance, risk factors and clinical applications of heart rate variability.