Precise quantification of physical activity is necessary in order to evaluate the relationship between physical activity and various types of disease and/or risk factors associated with disease. With the emergence of the study of the origins of coronary heart disease risk factors in children, it is imperative that accurate measurement of physical activity in the pediatric population be obtained. This review critically evaluates various field measures of physical activity in children including motion sensors, questionnaires, and heart rate. A new heart-rate quantification procedure is also presented that may provide an index of the quality of physical activity.