There are many systems on aircraft that need to be controlled and/or monitored, either manually by the crew, or automatically. A switch provides the simplest form of control and monitoring. For example, the crew need to know if any doors are not closed as part of their pre-flight check; on larger aircraft the position of control surfaces is displayed in the cock-pit. Using these two examples, doors can be either open or closed; control surfaces can move through an infinite number of positions (within their normal limits of travel). Many other aircraft parameters need to be measured, e.g. temperature and pressure. Measurements are made by a variety of transducers; these are devices used to convert the desired parameter, e.g. pressure, temperature, displacement, etc. into electrical energy. This chapter describes generic controls and transducer devices used on aircraft.