It is proven that wave run up on rough slopes is underestimated in small scale model tests due to scale / model effects. Given this fact, the same effects are suspected to be present for wave overtopping. A thorough comparison between prototype measurements of wave overtopping at three different coastal structures and scale model tests of these structures has been performed. The present paper gives the results from this comparison and presents a procedure to determine whether scale effects can be present in a certain situation. The procedure allows to calculate the magnitude of the scale and wind effect that can be expected. Since a lot of designs are based on physical model tests, this procedure has a broad range of applicability and its importance should be stressed.