Recently, interest in vertical breakwaters has grown. The development of ports from natural small harbours to artificial large harbours facing the outer sea has demanded the construction of breakwaters in rougher seas. The vertical breakwater could play a role of importance in this development. To obtain general guidelines for the design of vertical breakwaters, an European research project has been started, called PROVERBS-Mast Ill. This study is part of this project. Breakwaters are constructed to provide a calm basin for ships and to protect harbour facilities. For ports open to rough seas, breakwaters play a key role in port operations. One of the endangers of harbour tranquillity is wave transmission. Waves hitting the breakwater transfer part of their energy into the harbour, whether by energy through the breakwater, or by energy transfer over the breakwater if the run-up of the waves exceeds the top level of the breakwater. Goda (1969) has proposed relations describing wave transmission at vertical breakwaters, based on regular wave tests. Since then, little research work has been addressed to this subject. This report is an attempt to contribute to the enhancement of the understanding of wave transmission at vertical breakwaters. Data of various caisson breakwater types are discussed and analysed. The sloping top caisson breakwater gives more wave transmission than conventional and horizontally composite breakwaters. For horizontally composite breakwaters, wave transmission is in general less than for conventional breakwaters. The difference in wave transmission between conventional, parapet and perforated breakwaters is not significantly large. The applicability of the relations derived by Goda (1969) for irregular waves using the significant height, has been verified. These relations can also be used to describe wave transmission for various types of caisson breakwaters by adaptation of the coefficients. A relation between the coefficients and a parameter characterising the type of caisson breakwater could not be found. Wave transmission is due to overtopping and transmission through the structure. An effort is made to describe wave transmission due to overtopping with the percentage of overtopping waves. A relation between wave transmission due to overtopping and the percentage of overtopping waves, which depends on the crest freeboard, has been derived. A relationship for wave transmission through the structure is also given. These relations are based on data of a conventional caisson breakwater, i.e. caisson placed on a rubble mound foundation. The results are discussed and applied to sloping top caisson breakwaters. Finally, wave transmission has been studied in a theoretical approach to get a better insight in the process. The method as described in this report, however, consequently overestimates the wave transmission coefficient, probably due to the non-linearity of the phenomenon wave transmission.