This document discusses heart rate variability (HRV) and its importance in clinical applications. It provides background on HRV, noting that small variations in the time interval between heartbeats can provide information about a person's health. The document outlines several techniques used to analyze HRV, including measuring the variation in beat-to-beat intervals from electrocardiogram (ECG) readings. It also lists many conditions that can be evaluated through HRV analysis, such as cardiac health, stress, fatigue, arrhythmias, and rejection in organ transplant patients. In closing, the document emphasizes that properly quantifying and interpreting HRV patterns is important for clinical applications.