A flame detector is provided which comprises a housing (1), a test source of electromagnetic radiation (4) and a sensor (7). The source of electromagnetic radiation (4) and the sensor (7) are mounted within the housing (1). The source of electromagnetic radiation (4) is arranged to direct its output onto the sensor (7). The source of electromagnetic radiation (4) is arranged to emit radiation which simulates a flame. In this way, a means is provided within the housing (1) of the flame detector to test the flame detector without the need for an external test source, such as a test fire or a bulky and expensive test torch.