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The widening range of scientific fields currently contributing to this topic reflects the recognition that physical activity is embedded and intricately connected to every aspect of daily life. No longer viewed only as distinct and prolonged bouts of “vigorous physical exercise,” 6 physical activity is recognized as encompassing the accumulation of movement occurring throughout the day, regardless of location, type, or purpose. This broader view of physical activity complicates the study, understanding, and discussion of this key health behavior. The purpose of this chapter is to provide a brief discussion of Part C. Background and Key Physical Activity Concepts 2018 Physical Activity Guidelines Advisory Committee Scientific Report C-3 physical activity-related terminology and issues that may help readers understand the concepts, evidence, and interpretations that are presented elsewhere in this report. The Physical Activity Guidelines Advisory Committee Report, 20087 and the 2008 Physical Activity Guidelines for Americans8 demonstrated that the importance and value of physical activity and public health had been recognized at the highest level of government. The 2018 Scientific Report is further evidence of the importance of physical activity to the national interest. PHYSICAL ACTIVITY TERMS AND DIMENSIONS As the field has matured and the complexity of physical activity has become more apparent, applying clear definitions and descriptions of relevant concepts and issues has become increasingly important. In this document, the Committee has endeavored to use the most appropriate terms for the physical activity behaviors and concepts being discussed. Core Terms Physical activity is bodily movement produced by skeletal muscles that results in energy expenditure.9 The term, physical activity, does not require or imply any specific aspect or quality of movement. The term encompasses all types, intensities, and domains. Although the term “physical activity” has been used often as a short-hand description for moderate-to-vigorous-intensity forms of physical activity, given current interest and discussions about physical activity of intensities less than moderate-intensity.