A jet fire is a turbulent diffusion flame resulting from the combustion of a fuel continuously released with some significant momentum in a particular direction. Source momentum, and hence directionality distinguishes a jet fire from a pool fire. A second distinction is the absence in jet fires of any direct feedback from the fire to the source. The latter means that the evolution of their behaviour in compartments may be different from that of pool fires. It also means that jet fire hazards can be treated more deterministically because at least the mass flow rate and its behaviour with time is determined to a first order by the leak characteristics, the inventory and the blowdown characteristics.