SummarySynopsis:Clinicians should direct greater attention to positive health, with particular reference to the role of physical activity, in optimising the well-being of a child. Vigorous physical activity slows maturation, particularly if there is associated energy imbalance or emotional stress, but it does not greatly affect body size. Well-designed exercise programmes enhance the immediate physical, psychomotor and intellectual attainments of a child. Long term health benefits depend on a continuation of the physical activity. Internal motivation and the creation of positive attitudes are thus important. Acute illness is not prevented by vigorous exercise, but it has a favourable effect on the course of many chronic diseases. There are certain risks, including physical injury, psychological stress, and (occasionally) cardiac deaths, but in general, an exercise regimen enhances well-being, favouring the balanced development of a child.Concept of Health:Clinicians have traditionally viewed child health in terms of a steady weight gain and adherence to immunisation schedules. Interest in the broader World Health Organization definition of health was stimulated by the demonstration of poor physical performance in US children relative to their European counterparts, together with documentation of deteriorating oxygen transport in Canadian boys over the period from primary school to adolescence.Optimal Growth and Development:Comparisons with sedentary subjects have sometimes (but not always) shown that child athletes have slow growth, retardation of bone development, and (in girls) a delay of menarche. However, factors potentially confounding this comparison include the selection of athletes by body build, an inadequate energy intake in groups such as gymnasts, and the excitement of certain types of competition. A recent controlled study found some delay in wrist maturation, but no difference of overall size growth when selected classes at 2 primary schools were given an additional 5 hours of physical education per week.Well-being:There is lack of agreement on the extent to which the physical potential of a child can be enhanced by required programmes of physical activity. Negative results probably reflect inadequate training programmes, selection of initially fit subjects, or a masking of response by seasonal variations in physical condition. Gains of physical con-dition improve body image and thus the immediate well-being of a child. However, long term health benefits depend upon the faithful continuation of training sessions into adult life. There is no strong evidence that trainability is enhanced during adolesence, or that lack of training cannot be made good as an adult.