A clear difference between full-scale wave run-up measurements and small-scale model test results had been noticed during a MAST II project. This finding initiated a thorough study of wave run-up through the European MAST III OPTICREST project. Full-scale measurements have been carried out on the Zeebrugge rubble mound breakwater. This breakwater has been modeled in three laboratories: two 2D models at a scale of 1 : 30 and one 3D model at a scale of 1 : 40 have been built at Flanders Hydraulics (Belgium), at Universidad Politécnica de Valencia (Spain), and at Aalborg University (Denmark). Wave run-up has been measured by a digital run-up gauge. This gauge has proven to measure wave run-up more accurately than the traditional wire gauge. Wave spectra measured in Zeebrugge have been reproduced in the laboratories. Results of small-scale model tests and full-scale measurement results have been compared. This comparison confirmed the MAST II outcome: wave run-up is underestimated by small-scale model tests. The difference between full-scale measurement results and small-scale model test results is the result of model effects. The porosity of the armor layer has a significant influence on wave run-up and may explain the dependency of wave run-up on the water level observed in Zeebrugge. An influence of the spectral shape has also been noticed.