Fitness trackers are a group of devices including watches, phones, and rings that track physical activity. JAMA Executive Editor Gregory Curfman, MD, speaks with I-Min Lee, MBBS, ScD, professor of medicine at Harvard Medical School and professor of epidemiology at Harvard T.H. Chan School of Public Health, about using fitness trackers to guide advice on activity prescription. Related Content: Fitness Trackers to Guide Advice on Activity Prescription