Modern monitoring of sport and exercise is being driven by developments in technology, particularly electronics and microelectromechanical systems. The data consumer of the sports monitoring device, usually the athlete and the support staff, determines the sophistication, the ease of use and the system outputs. The core monitors are the sports monitoring devices that are in widespread use, either across a broad spectrum of sports or in particular sports. Heart rate monitors and pedometers are popular for many reasons, but the primary reasons are that they are simple to operate and simple to understand, and they provide an immediately useful output. Physiological monitoring is considered by many people to be the forte of the heart rate monitor, but in many large studies the most common parameter of interest is the estimated energy expenditure of the subject. The tool selected for this task is the accelerometer.