Abstract: The fire detector comprises an insertable detector assembly which includes a sensor arrangement (2), an electronic evaluation system and a housing (3) which surrounds the sensor arrangement (2) and has openings to provide access by air and, when applicable, smoke to the sensor arrangement (2). The detector is of modular construction and is configured to accommodate detection modules having sensors for different fire parameters, all detection modules being compatible with a single housing (3). The detection module may be configured for optical, thermal or optical-thermal fire detection and/or for detecting combustion gases. The sensor arrangement (2) and the above-mentioned access openings are arranged substantially in one plane, whereby a shallow construction is achieved even in the case of a multi-criterion detector.