This article provides a review of the techniques that have been developed for measurement of physical activity in children and adolescents. Physical activity has been measured in youngsters using three major approaches: direct observation, heart rate or motion monitors, and self-report measures. Each of these techniques has relative benefits and deficiencies. Direct observation is a valid procedure requiring little interference and can provide good information concerning contextual variables (i.e., physical and social environment). However, direct observation is costly in terms of investigator and observer time. Motion sensors and heart rate monitors overcome problems associated with subject recall of activity and are less costly than direct observation. However, these instruments can be prone to technical problems and they provide no information concerning specific activities or the context in which activities are performed. Self-report and proxy report (e.g., by parents or teachers) techniques are relatively inexpensive, but their validity is limited by the ability of the subject (or proxy) to recall and report activity behavior. A major limitation in this field has been the lack of a "gold standard" measure of energy expenditure. The lack of such a criterion measure has precluded validation of the aforementioned activity measures as indicators of energy expenditure in children and youth.