Physical activity has far-reaching benefits for physical, mental, emotional, and social health and well-being for all segments of the population. Despite these documented health benefits and previous efforts to promote physical activity in the U.S. population, most Americans do not meet current public health guidelines for physical activity. Surveillance of physical activity is a core public health function that is necessary for monitoring population engagement in physical activity, including participation in physical activity initiatives. However, physical activity is challenging to assess because it is a complex and multidimensional behavior, and the lack of surveillance systems is a critical gap.