The subject of this dissertation is flooding in lowland areas caused by intense precipitation. The probability of flooding will increase when we do not adapt our water systems. For example because of expected changes in climate and the consequences of subsidence, sea level rise, and urbanization. While the probability of flooding will increase, the degree of acceptance by Dutch society of possible flooding is decreasing. Society expects from the water manager that it may count on "dry feet". Within this context the task of water boards is to determine whether water systems are still suitable to provide sufficient protection against flooding. If not, the boards have to determine which improvements are necessary to raise the level of protection. However, nowadays more scenarios need to be incorporated in the analysis. This leads to the question whether the classic methods most commonly applied need to be adjusted or not. The thesis of this dissertation is that assessing the urgency to adapt an existing water system needs to be based on risk analysis. Next, identification of improvements should be based on cost/benefit analysis, because (1) this approach enables the specific local circumstances to be taken into account better than the present standards do; (2) it provides insights in the consequences of all kinds of future scenarios in a unit everyone can understand: money; and (3) the analysis supports a more transparent decision making process since it forces decision-makers to make explicit why it is attractive to take certain measures.