Temperature controllers are used to maintain a constant temperature (the target) in furnaces and other equipment. Generally, they include a measured input indicator and a setpoint controller. They generate a control signal according to the difference between the indicated value (measured input) and target value, and adjust the temperature to match the target.

To measure the temperature, engineers can connect a variety of sensors such as thermocouples or RTDs. To adjust the temperature, the temperature controller can send relay output, current output (4–20 mA DC), or other kinds of output to the control element (such as a heater or valve). Temperature controllers can also be used in non-temperature applications such as flow or pressure, and such devices are called indicating controllers.