||Large individual differences exist in aerobic fitness in childhood and adolescence, but the relative contribution of genetic factors to this variation remains to be established. In a sample of adolescent twins and siblings (N = 479), heart rate (HR) and maximal oxygen uptake (V?O2max) were recorded during the climax of a graded maximal exercise test. In addition, V?O2max was predicted in two graded submaximal exercise tests on the cycle ergometer and the treadmill, using extrapolation of the HR/V?O2 curve to the predicted HRmax. Heritability estimates for measured V?O2max were 60% in mL/min and 55% for V?O2max in mL/min/kg. Phenotypic correlations between measured V?O2max and predicted V?O2max from either submaximal treadmill or cycle ergometer tests were modest (.57 < r < .70), in part because of the poor agreement between predicted and actual HRmax. The majority of this correlation was explained by genetic factors, therefore the submaximal exercise tests still led to very comparable estimates of heritability of V?O2max. To arrive at a robust estimate for the heritability of V?O2max in children to young adults, a sample size weighted meta-analysis was performed on all extant twin and sibling studies in this age range. Eight studies, including the current study, were meta-analyzed and resulted in a weighted heritability estimate of 59% (mL/min) and 72% (mL/min/kg) for V?O2max. Taken together, the twin-sibling study and meta-analyses showed that from childhood to early adulthood genetic factors determine more than half of the individual differences in V?O2max.