A fire detector includes a sensor (4) for, in use, detecting electromagnetic radiation emitted by a flame and generating an electronic signal corresponding to the detected electromagnetic radiation. The detector further includes a processing device (2) configured to select a portion of the electronic signal corresponding to a first time period and compare the selected signal portion with at least one parameter to determine an initial presence or absence of a flame, the processing device being further configured, upon determining the initial presence of the flame, to select a further portion of the electronic signal corresponding to a further time period and comparing the further selected signal portion with at least one parameter to determine a definite presence or absence of the flame.