Background Despite the increasing interest in cardiac autonomic nervous activity, the normal development is not fully understood. The main aim was to determine the maturation of different cardiac sympathetic-(SNS) and parasympathetic nervous system (PNS) activity parameters in healthy patients aged 0.5 to 20 years. A second aim was to determine potential sex differences. Methods and Results Five studies covering the 0.5- to 20-year age range provided impedance- and electrocardiography recordings from which heart rate, different PNS-parameters (eg, respiratory sinus arrhythmia) and an SNS-parameter (pre-ejection period) were collected. Age trends were computed in the mean values across 12 age-bins and in the age-specific variances. Age was associated with changes in mean and variance of all parameters. PNS-activity followed a cubic trend, with an exponential increase from infancy, a plateau phase during middle childhood, followed by a decrease to adolescence. SNS-activity showed a more linear trend, with a gradual decrease from infancy to adolescence. Boys had higher SNS-activity at ages 11 to 15 years, while PNS-activity was higher at 5 and 11 to 12 years with the plateau level reached earlier in girls. Interindividual variation was high at all ages. Variance was reasonably stable for SNS- and the log-transformed PNS-parameters. Conclusions Cardiac PNS- and SNS-activity in childhood follows different maturational trajectories. Whereas PNS-activity shows a cubic trend with a plateau phase during middle childhood, SNS-activity shows a linear decrease from 0.5 to 20 years. Despite the large samples used, clinical use of the sex-specific centile and percentile normative values is modest in view of the large individual differences, even within narrow age bands.