This study has been undertaken in order to find out what is to be understood by " rather gentle " slopes. That is, it is an inquiry into the limits of applicability of the refraction - diffraction equation. In this study, the refraction - diffraction equation, as well as the set of equations from which it has "been derived, are used to predict the values of the wave potential at the water surface. The computation is done numerically, using A Finite Element Package ( " AFEP " ) which has been developed at Delft University. The predictions by the two models are compared for identical cases and a correlation between the slope of the bottom and the discrepancy in the predictions is sought. Only simple linear waves are considered in this study.