Heart rate variability (HRV) reflects the modulation of cardiac function by autonomic and other physiological systems, and its measurement from ambulatory electrocardiograph (ECG) recordings is a useful method for both clinical and scientific purposes. Heart rate variability can be measured by several linear and non-linear methods, and various methods can give different information on neural and other physiological influences on the heart. Heart rate variability is abnormal in various settings of ischaemic heart disease, and the most important current application of HRV analysis in clinical cardiology is its measurement in postinfarction patients, in whom abnormal HRV indicates an increased risk of cardiac mortality. Future research may expand the clinical utility of HRV measurement to other clinical situations.