The central issue for authorities (as well as the public) is how and when to respond to forecasted extreme water levels on rivers, lakes and along the coast and large-scale flooding is an actual risk. The decision-making process is influenced by contradicting information, overloads and gaps in information, rumours, uncertainties in forecasts, the consequences of a flood and the effectiveness of measures. Emergency measures can be taken to reduce the probability of flooding(e.g. placing sand bags), other measures can be taken to reduce the consequences of a flooding (such as evacuation of inhabitants). For many of these measures, decisions are made days or hours prior to the expected moment of occurrence of the flooding. Using forecasts of water levels, by definition uncertain, and forecasts of the strength of levees, decisions can be made based on the acceptability of the actual flood risk level. The concept of risk can be used to prioritise measures in case of limited time.