There are well-established chronic disease prevention and broader public health benefits associated with being physically active. However, large proportions of the adult populations of developed countries and rapidly urbanizing developing countries are inactive. Additionally, many people’s lives are now characterized by large amounts of time spent sitting—at work, at home, and in automobiles. Widespread urbanization, particularly in low- and middle-income countries, has resulted in large segments of traditionally active rural populations moving into cities. Many previously manual tasks in the occupational and household sectors have become automated, making life easier and safer in many respects. However, a pervasive consequence of these developments is that large numbers of people globally are now going about their daily lives in environments that place them at risk of overweight and obesity, type 2 diabetes, cardiovascular disease, and other health problems. This represents a formidable set of public health challenges. In this context, the present chapter describes: key definitions and distinctions relating to physical activity, exercise, and sedentary behaviour; current evidence on relationships physical inactivity and sitting time with health outcomes, and associated public health recommendations; prevalence and trends in physical activity and sedentary behaviours, and some of the key issues for surveillance and measurement; and, how physical activity and sedentary behaviour may be understood in ways that will inform broad-based public health approaches. An interdisciplinary and intersectoral strategy is emphasized. This requires working with constituencies beyond the public health field, such as urban planning, architecture, occupational health and safety, and social policy.