Earlier, it has been shown how to estimate the probability of an accident based on historical accident numbers. The simplest and most intuitively correct manner is to base accident frequency estimates on exposure criteria such as vessel-years (i.e. number of vessels at risk per year). However, such a statistical approach may only describe the mean risk of a large number of ships and not reflect variation in technical standards, environmental conditions and traffic density. In certain instances the analysis of risk will be undertaken for specific fleets or for certain waters or fairways. This demands another method than the statistical approach based on fleet-year exposure. In this chapter it will be shown how the probability of an impact-type accident can be estimated for a specified seaway. By an impact-type accident we mean collision, grounding, stranding or allision (above-water object impact).