In common with other volumes in the Essential Knowledge series from MIT Press, this short book provides concise, cogent, current information on the state of play of a specific technical area. While the IoT is still at an early stage of development, it is clear that the emergence of mass market technologies that allow seamless communication between smartphones and smartwatches, health and activity monitors, embedded web servers, network-aware sensors and radio-frequency identification (RFID) tags in products will make such systems close to ubiquitous in a relatively short time.