Sea walls are expensive, and fixing a seawall freeboard at too large a value has a financial penalty and is unnecessarily damaging to the natural environment owing to the increased impact of the structure on its surroundings. On the other hand, if the crest of a sea wall is set too low then there are potential problems with structural safety and flooding from wave overtopping. Hence, it is important to strike the correct balance between satisfying the structural and functional requirements of the project, avoiding unnecessary expense, and having undesirable impacts on the surrounding environment. The prediction of wave overtopping rates is usually based on empirical equations fitted to laboratory data. These equations do not have any theoretical basis. However, a new model has now been developed which, unlike existing expressions, accounts for the fact that no overtopping (apart from wind-blown spray) occurs if the sea-wall freeboard exceeds the maximum wave run-up on the face of the structure. This fact is of practical importance because allowable overtopping discharges to ensure the safety of people and property are quite small. This paper starts with a brief review of existing overtopping equations, then presents the new model, and concludes by giving an example of its practical implications. It is shown that, for some conditions, the new model predicts sea-wall freeboards which are several metres less than those predicted by the well-known expression given by Owen.