In recent years, there has been tremendous growth in the use of wearable activity trackers in biomedical research. Activity trackers are also becoming more popular with consumers, who are able to share their data with researchers and practitioners. Steps per day is a useful variable that is estimated from most wearable activity trackers. It has intuitive meaning, is strongly associated with health variables, and has the potential to be standardized across devices. Activity trackers and other wearable medical devices could provide new information on health-related behaviors and their interaction with genetic and environmental variables. If integrated into medical practice, wearable technologies could help motivate patients to change their health behaviors and might eventually be used to diagnose medical conditions. The convergence of wearable medical devices, computer applications, smart phones, and electronic medical records could influence the practice of lifestyle medicine.