This paper deals with model tests conducted at the Delft Hydraulics Laboratory (DHL) and the River and Harbour Laboratory at the Technical University of Norway (RHL) for the design of the Europoort Breakwater. A series of tests with regular waves was conducted at the DHL from which the design of the breakwater was decided. The chosen design was tested with irregular waves based on in situ observations. Wave spectra, wave height distributions and the joint distribution of wave height and period were specified. These tests were conducted at the RHL, and some tests were repeated at the DHL. It has been commonly assumed that the destructive effect of a train of regular waves on a breakwater in model is equal to the effect of irregular waves with a significant wave height corresponding to the height of the regular waves. The tests showed that for this particular breakwater the irregular waves represented a more severe wave attack than the regular waves.