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 in public health is the ongoing systematic collection, analysis, and interpretation of outcome-specific data, which can then be used for planning, implementation and evaluation of public health practice. 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. Surveillance activities are guided by standard protocols and are used to establish baseline data and to track implementation and evaluation of interventions, programs, and policies that aim to increase physical activity. However, physical activity is challenging to assess because it is a complex and multidimensional behavior that varies by type, intensity, setting, motives, and environmental and social influences. The lack of surveillance systems to assess both physical activity behaviors (including walking) and physical activity environments (such as the walkability of communities) is a critical gap. Implementing Strategies to Enhance Public Health Surveillance of Physical Activity in the United States develops strategies that support the implementation of recommended actions to improve national physical activity surveillance. This report also examines and builds upon existing recommended actions.