Fire safety in tunnels has become a major international issue, following catastrophic tunnel fires that have occurred worldwide. As a result, it has become essential to gain a much better understanding of all aspects of tunnel fire safety, from basic science to acceptable risk and the law. This need has produced a significant increase in research and investigation. The fundamental principles which emerge from this, form the bed-rock for decision making on how tunnels may be designed, or up-graded and operated in an acceptable way.