Instrumentation in chemical facilities plays an essential role when implementing safety devices. Sensors communicate the most important physical values either to a control loop to regulate a process (including its shutdown), or to a simple display. Sensors are also a key component of Safety Instrumented Systems (SIS). This Unit addresses the principle of a control loop and the role of sensors. Sensors must be chosen carefully according to the context of their use. The various characteristics enabling the appropriate selection of the following sensors are studied: temperature sensors, pressure sensors, flow rate sensors, and level sensors.