Abstract: A detector unit for use in a fire detection system is provided, comprising: a sensor for monitoring an environmental condition; an indicator for generating a visible indication; and a controller in connection with the sensor and the indicator, wherein the controller is configured to communicate with a fire alarm control panel of the fire detection system; wherein the controller is configured to operate the detector unit in a testing mode, the testing mode being for testing a response of the fire detection system to an environmental condition that is indicative of a fire; and wherein the controller is configured to cause the indicator to indicate an outcome of a test carried out on the detector unit when the detector unit is operating in the testing mode.