The human body serves as one of the best examples of a complex system that contains a wide variety of sensors capable of sensitively and selectively detecting a wide variety of quantities or measurands. The most familiar sensors in the human body are those that relate to vision, hearing, smell, taste, and feel. The sensors within the human body, particularly the five principal sensors, can be looked upon as real-time control systems. It is essential that these control systems communicate accurately and effectively so the human body can avoid dangers and perform satisfactorily. In a real-world system, such as an automobile, the ability of an electronic control system to communicate accurately and effectively to the automobile operator is critical to the operation of the automobile. The sensor is by its nature either an active or passive device that serves as the heart of a measurement system.