This book is about wearable sleep technology. Sleep assessment technologies, or “sleep trackers”, that can be used outside of the laboratory are becoming increasingly popular. They are utilized in scientific research, among behavioral sleep medicine clinicians, and by members of the general public who are interested in learning more about their sleep. There are few guidelines currently available to help users discern which device is most suitable for their needs. Bold marketing claims often suggest that devices are more accurate than they are. By understanding the history of how these devices were developed, the limitations of the current proxy measures of sleep, and the inherent bias in validation studies of the most popular devices used today, readers can learn about what to realistically expect from device performance. This book comprehensively explains how sleep trackers work, how they can be evaluated by users, and how they can be used in practice.