In the frame of the European Commission co-funded Network of Excellence HySafe (Hydrogen Safety as an Energy Carrier, www.hysafe.org), five organizations with significant experience in explosion modelling have performed numerical simulations of explosions of stoichiometric hydrogen-air mixtures in a 78.5 m long tunnel. The five organizations are the Karlsruhe Research Centre, GexCon AS, the joint Research Centre, the Kurchatov institute Research Centre and the University of Ulster. Five CFD (Computational Fluid Dynamics) codes with different turbulence and combustion models have been used in this Standard Benchmark Exercise Problem (SBEP). Since tunnels are semi-confined environments, hydrogen explosions in tunnels can potentially be critical accident scenarios from the point of view of the accident consequences and CFD methods are increasingly employed to assess explosions hazards in tunnels. The objective of the validation exercise is to assess the accuracy of the theoretical and numerical models by comparisons of the simulation results with the experimental data. A very good agreement between experiments and simulations was found in terms of maximum overpressures. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.