The overall purpose of this project is to develop decision aids and methodological guidelines for performing risk assessments of low-probability, high-consequence events at large-scale technological facilities. The decision aids and methodological guidelines will cover (1) methods for identifying combinations of failures that could lead to high-consequence accidents; (2) methods for estimating the probabilities of such accidents (especially when there is no historical experience upon which to base such estimates); and (3) methods for estimating the health, safety, and environmental consequences of such accidents. Detailed case studies will be used to examine and compare the strengths and weaknesses of alternative decision aids and methodological approaches.