Within the framework of the European project CLASH phenomena related to wave overtopping are studied. One of the topics that require further attention is the influence of low-frequency waves on wave overtopping. Here, use is made of field measurements performed at the Petten Sea-defence (by Rijkswaterstaat and made available by RIKZ) and numerical models. During the field measurements storm conditions have been measured. In the present study a series of conditions within the storm period of October 25-28, 2002, have been selected. Field measurements were analysed to obtain incident waves from the measured surface elevations. This analysis shows that the dike reflects low-frequency waves up to 100% (on average 61%), while wave reflection coefficients based on the energy in the short waves are on average 23%. Two numerical models have been applied, one for wave propagation over the foreshore and one for the wave motion on the dike. The model for wave propagation over the foreshore TRITON has been used to study whether the amount of low-frequency energy can be computed accurately and it has been applied to obtain wave conditions at the toe of the dike. Time-signals from measurements and computations have been used as input for the numerical model ODIFLOCS that can simulate wave overtopping. Because the Petten Seadefence is so high that no wave overtopping occurs, in the computations a crest lower than the actual crest has been applied. The results of this analysis indicate that for the studied storm conditions low-frequency energy increases the mean overtopping discharge with a factor up to a maximum of 5. This increase depends on the percentage of low-frequency energy. For the analysed storm period ‘October 2002’, a much lower factor was found (up to 1.4). Also for a condition considered as a ‘super storm condition’ a factor of 1.4 was found. A factor of 1.4 would have a rather small effect on the required crest elevation due to the influence of low-frequency energy.