Seiches affecting the Port of Rotterdam are generated in the Southern North Sea. Their generation is investigated with observations and numerical simulations. A wavelet analysis of the observations, both at sea and in the harbor, shows that low-frequency energy (0.1--2.0 mHz) does indeed occur at sea prior to each seiche event in Rotterdam. An analysis of 6 years of weather charts indicates that all 51 seiche events in this period (with amplitude exceeding 0.25 m) coincided with the passage of a low-pressure weather system. Some of these low-pressure systems included a sharp cold front (classical or ana), whereas others included a more diffuse cold front (split or kata). Numerical simulations with a hydrodynamic model driven by meteorological observations reproduced the seiches for situations with the sharp cold fronts correctly. The seiches that were simulated for situations with the gradual cold fronts do not agree with the observations, which is ascribed to the inadequate atmospheric forcing of the hydrodynamical model.