Physical activity is important for the prevention of chronic disease morbidity and mortality, and the lack of adequate levels of physical activity represents a growing public health burden around the world. The purpose of this report is to introduce the concept of the "Physical Activity Transition" and to explore the potential effects that declining physical activity levels may play on health and life expectancy as countries undergo economic and demographic changes. Physical activity is related to mortality rates in humans, and the available evidence suggests that the adoption of a lifestyle characterized by lower levels of physical activity will attenuate the expected gains in life expectancy associated with the epidemiological transition. Advances in the measurement of physical activity at work, in the home, for transport, and in leisure time in a wide variety of populations will be integral to advancing the current understanding of how macro-level factors shape physical activity patterns and patterns of morbidity and mortality.