Regular physical activity has been recommended for decades because of the notable health benefits, while the potential negative consequences of sedentary activity are only beginning to be understood. This disconnect may be due in part to early assumptions that physical activity and sedentary activity were inversely related, such that regular engagement in physical activity was incompatible with high amounts of sedentary activity. More recent evidence suggests that physical activity and sedentary activity are distinct, and that excessive sedentary activity may be detrimental to health and well-being. Despite the known health benefits, worldwide participation in regular physical activity is low. High rates of sedentary activity are also prevalent, particularly in developed countries. Common theories for understanding physical and sedentary activity are articulated, as well as effective interventions for increasing physical activity and decreasing sedentary activity. Technological approaches offer a novel and potentially more sustainable approach to intervening on physical and sedentary activity. Measuring physical and sedentary activity presents unique challenges, which are being addressed in part by the use of technology.