Various sensors are needed for the monitoring and control of airframe systems. The indications and/or control circuit need to be informed of the position of a particular feature on the aircraft; this is achieved by a position sensor. Broadly speaking, the sensors can be considered as detecting one of two states (or 'conditions'), or a variable position. Two state conditions include: landing gear (up or down) or cabin doors (open or closed). Variable positions include: control surfaces and flap position. Two state position devices are detected by micro-switches and proximity sensors; variable position devices are detected by a variety of devices including synchros and variable resistors. This chapter reviews airframe systems such as landing gear control and indication, control surface position and indicating systems.