The traditional factor of safety: a factor of safety against what?

Recent developments in the analysis of dams have led to a better insight and confidence in the behaviour of dams. However, the way we are practicing dam safety, using deterministic factors, has not changed very much. Factors of safety are round numbers, lumping together uncertainties and imperfect knowledge of various sources. Transparency is weak; tracking the main sources of uncertainties is less of an issue. Its influence is marginalized, as is the search for a balance between safety and economy. This paper explores steps accounting for uncertainties when evaluating the safety of dams.