Habitual physical activity in children has become a major field of interest for many exercise and sport scientists as well as for pediatric clinicians who specialize in the prevention of and rehabilitation from hypokinetic diseases. This paper discusses the state of the art concerning the measurement of habitual physical activity, identifies factors which may cause hypo-activity in children, and outlines specific problems on which research should be focused. The areas addressed include: the use of different field methods to measure physical activity and methodological constraints in young children; the level of daily activity in healthy children; and, finally, some aspects of reduced habitual physical activity in pediatric diseases.