The link between the sympathetic-parasympathetic imbalance and the risk for arrhythmias led to the development of methods for assessing the autonomic nervous system activity. Currently, the most widely used method is heart rate variability, a readily available non-invasive tool. Heart rate variability measures the variations in time intervals between consecutive heartbeats, indicating the interaction between the sympathetic and parasympathetic nervous systems. In addition to its established roles in cardiology and diabetes, its clinical applications have expanded in recent years to encompass various health conditions and diseases. This book offers a comprehensive overview of heart rate variability, detailing its clinical relevance and recent advances in this rapidly evolving field.