The decline in physical activity with age may be the most consistent finding in physical activity epidemiology. Although this phenomenon is well accepted, it is not well understood. The purposes of this symposium are to determine whether there are critical periods of decline and quantify sex differences in the decline. Data from cross-sectional and prospective studies indicate the decline is steepest between the ages of 13 and 18. The decline is generally greater for male than female subjects, and the decline varies by type and intensity of the activity. A review of animal studies documented the age-related decline in many species. This suggests a biological basis, and a probable mechanism is the dopamine system that regulates motivation for locomotion. The decline in physical activity with age is antithetical to public health goals, so methods of countering the decline need to be developed, based upon an improved understanding of the phenomenon and its causes.