The selection of the most psychometrically appropriate self-report tool(s) to measure specific physical activity constructs has been a challenge for researchers, public health practitioners, and clinicians, alike. The lack of a reasonable gold standard measure and inconsistent use of established and evolving terminology have contributed to these challenges. The variation of self-report measures and quality of the derived summary estimates could be attributed to the absence of a standardized conceptual framework for physical activity. To present a conceptual framework for physical activity as a complex and multidimensional behavior that differentiates behavioral and physiological constructs of human movement. The development of a conceptual framework can provide the basic foundation from which to standardize definitions, guide design and development of self-report measures, and provide consistency during instrument selection. Based on our proposed conceptual framework for physical activity, we suggest that physical activity is more clearly defined as the behavior that involves human movement, resulting in physiological attributes including increased energy expenditure and improved physical fitness. Utilization of the proposed conceptual framework can result in better instrument choices and consistency in methods used to assess physical activity and sedentary behaviors across research and public health practice.