Diffraction of both regular and irregular waves by a Concrete Gravity Sub-structure (CGS) was investigated using both experimental surface elevation measurements and computational results of the linear diffraction code DELFRAC. The influence of the box-shaped base that supports the four vertical columns was studied independently from the columns, using data from regular wave tests on the Malampaya CGS. DELFRAC was shown to give accurate results for the focusing of waves over the underwater structure. Results from a regular wave data analysis of model tests on the complete Sakhalin II project Lunskoye structure were compared to those predicted by the linear diffraction code. For the wave cases tested, the first-order amplitudes were accurately predicted. Diffraction of irregular waves was studied in a similar way and linear diffraction theory for random seas gave an excellent prediction of incident wave spectral diffraction, including the peaks in the diffracted spectrum near twice the peak in the input spectrum. The results obtained for the Lunksoye structure in the present study are consistent with results found in similar studies on less complex structures. An attempt to predict the extreme crest heights from the diffracted spectrum was made using a Weibull distribution and a second-order expansion of the sea surface that captures the effects of wave steepness, water depth and directional spreading with no other approximation than the truncation of the expansion at second order. Depth induced breaking appeared to be an important phenomenon limiting the crest heights.