The fitness and physical activity levels of children and youth are commonly questioned, but the evidence cited is both equivocal and methodologically diverse. The amount and type of physical activity undertaken during childhood that is appropriate for optimal health is unknown, although it has been suggested that, in the absence of such criteria, activity levels known to confer health benefits in adults are also appropriate for children. The measurement of activity in children is problematical, and there is currently no valid method of assessing activity levels that is feasible for use in large studies. Therefore, studies may lack either internal validity or wider applicability. Studies using self-report methods indicate relatively high levels of activity with 60 to 70% of children taking sufficient 'appropriate' physical activity. However, a variety of activity thresholds have been used. Studies that use more objective methods report much lower levels of activity, especially when cardiovascular fitness criteria are applied. The use of less stringent health-related thresholds results in higher levels of 'appropriate' activity. Nearly all studies of teenagers report a decline in activity with age during this period. Data from the large population studies indicate that activity levels peak in children at around 13 to 14 years of age, and then markedly decline. Boys are normally reported to be more active than girls, but this difference is greatly reduced when moderate activity alone is compared, indicating that boys participate in more vigorous exercise than girls. The health effects of low levels of vigorous activity in children are unclear.(ABSTRACT TRUNCATED AT 250 WORDS)