It is now known that nearly half of the toll that illness takes in developed countries is linked to four unhealthy behaviors: smoking, excess alcohol intake, poor diet, and physical inactivity. These common risk behaviors cause preventable, delayed illness that then manifests as chronic disease, requiring extended medical care with associated financial costs. Chronic disease already accounts for 75% of U.S. health-care costs, foreshadowing an unsustainable financial burden for the aging population [1]. We are facing an urgent need to re-engineer health systems to improve public health through behavior change, and technology-supported behavioral change interventions will be a part of 21st-century health care. As new technical capabilities to observe behavior continuously in context make it possible to tailor interventions in real time, the way we understand and try to influence behavior will change fundamentally.