ABSTRACT: In this paper, the implications of integration of uncertainties in the design of flood protecting structures will be analysed. In particular the probability of inundation of the low-lying polders protected by dikes from the Dutch Lake IJssel (1200 km 2) is studied on the basis of a physical model. The uncertainty in the probability of overtopping is analysed with a first order reliability method (FORM). The FORM-calculations showed that the uncertainty in the wind speed and the uncertainty in the water level contributed most in the total uncertainty of the probability of overtopping of a dike with given height. A comparison has been made between the results of the FORM-calculations and the calculations where only intrinsic uncertainty has been assumed in the basic variables. Furthermore, in this paper, the reliability-based optimal design of the dikes along the lake is studied. Finally the policy implications of the uncertainty integration in the design of dikes are discussed.