The Fire Detection System for spacecraft has been designated as a subsystem of the ECLSS and is a little bit like a fish out of water, in that the main concern of the life support system designers does not lie in the field of fire protection and therefore draws little attention. Hazard detection and control does become of major concern however, when it comes to the protection of the spacecraft and personnel from fire damage.The design of the Space Shuttle's early warning Incipient Fire Detector is reviewed to explain the research that was conducted to establish its performance criteria. Comparisons are made to the Shuttle and Freedom fire detection specifications to illustrate the need for a better understanding of when is a hazard really a hazard and what defines early warning. A look is taken at the concern of detecting a fire hazard in microgravity. Is the lack of convective buoyancy forces that carry the products of combustion to the sensor location a problem or is the energy release and diffusion forces during the combustion phase sufficient?Finally, a description of a Fire Detection System concept for future spacecraft is given.