Heart rate variability is a recognized tool for the estimation of cardiac autonomic modulations. Recently, several studies have advanced the field of heart rate variability in three areas: 1) in technical modes of electrocardiogram processing and heart rate variability assessment, 2) in physiologic understanding and interpretation, and 3) in clinical and practical use. The most recent technical studies concentrate on the analysis of nonlinear aspects of heart period variations and on coherence between electrocardiogram variability and other physiologic factors such as respiration. Principal physiologic investigations studied the components of heart rate variability attributable to the individual limbs of the autonomic nervous system. It is becoming recognized that behavior and responses to the environment, including the psychosocial environment, play an important role in long-term heart rate variability. Established clinical applications of heart rate variability are presently restricted to the assessment of risk after myocardial infarction and to the early diagnosis of diabetic neuropathy. Nevertheless, recent studies indicate that heart rate variability might soon become a similarly powerful tool for risk prediction and monitoring of disease progression in congestive heart failure and for monitoring or reinnervation after heart transplantation.