Naturally occurring oscillations in heart rate have long been considered to reflect the modulating influences of the autonomic nervous system. Individual reports of heart rate variability in healthy and sick neonates and infants have provided valuable information as to pathophysiological changes in autonomic cardiovascular control. It is evident that prematurity and poor health are reflected in attenuated heart rate variability. However, at present, there are few standard criteria for the analysis of heart rate variability, preventing precise comparisons among the various studies. Until recommendations for standardized analytical methods are made, clinical application of heart rate variability in neonatal and infant prognosis and therapy remains premature.