This paper gives an overview of the tunnel fire research, both experimental and theoretical work with the exception of computer modelling. The overview shows that the critical velocity for longitudinal flow in a tunnel is the single most well investigated fire phenomenon found in the tunnel fire research literature. This knowledge has reached a high level but there are still some areas that need to be investigated further. For example, very little work has been done on backlayering of smoke upstream a fire and cross-sectional effects on heat release rates, temperatures, flame lengths, fire spread and toxic conditions. Extensive work using model scale tunnels has contributed with new information and clarified many phenomena but the lack of good large scale experimental data is and will be a key issue in near future. This concerns mainly the correlation between heat release rate and maximum ceiling temperatures, the influence of ventilation on maximum heat release rate and fire growth rate and fire spread in and between vehicles. The extensive theoretical work which has been conducted concerning the effects of ventilation on heat release rates needs to be further investigated in large scale tests.