The EXTREME SEAS project addresses assessment of extreme sea states, and ship behavior in such sea states. The project will study physical and statistical properties of extreme waves, and develop advanced numerical and physical simulation models for wave-structure interaction. A further objective is to develop warning criteria for marine structures against extreme sea states and extra-ordinarily large waves, and to implement them in a marine weather forecasting system operated by a meteorological office belonging to the Consortium. The developed warning criteria will be available for decision support systems for marine structures.