The proceedings include papers presented at the 11th International Symposium on Tunnel Safety and Security (ISTSS) held in Reykjavik on 9-11 April 2025. The symposium is well established in the tunnel fire community and the success of ISTSS is a tribute to the pressing need for continued international research and dialogue on these issues. The proceedings provide state-of-the-art knowledge in the field of fire safety and security in tunnels and other underground structures. We clearly see that fire dynamics is a major area of concern with a focus on smoke spread and smoke stratification especially for tunnels with natural ventilation, while fire and explosion hazards of alternative fuel vehicles and the use of water-based fixed firefighting systems continue to attract interest. Furthermore, evacuation, engineering design and risk assessment remain focused areas of many papers. This year there are also specific focuses on ventilation and emergency management. Numerous renowned researchers and engineers have contributed to these topics at this symposium for which we are grateful.