Abstract: Apparatus for testing the sensitivity of a smoke detector (12) in-situ, including a head assembly (1) comprising a housing (14) for surrounding a smoke detector in-situ and forming part of a sensing loop (14, 15, 16, 17)through which air carrying an aerosol will circulate, a sensor (25, 26, 27) for sensing the concentration of aerosol in the air circulating in the loop, a fan (20) for causing air to circulate in the loop, an aerosol container (24) for supplying aerosol to the loop and a control unit 2 for controlling the concentration of aerosol in the air circulating in the loop by controlling the fan (20) and the aerosol container (24) in response to the output of the sensor.