A self-monitoring smoke detector comprising a housing (11) defining an internal chamber (12). An optical transmitter (13) is mounted within the housing (11) so as to direct light into the optical chamber (12). An optical receiver (14) is mounted in the housing (11) and in optical communication with the optical chamber (12). The optical transmitter (13) and the optical receiver (14) are so positioned that light from the transmitter cannot directly reach the receiver. Monitoring means is provided, comprising first and second light-scattering means (13a and 14a) positioned respectively in alignment with the transmitter (13) and the receiver (14). The arrangement is such that, in the absence of reflector particles in the optical chamber, light from the transmitter (13) can reach the receiver (14) only after scattering at the first light-scattering means (13a) and then after scattering at the second light-scattering means (14a).