The floating breakwater BW400 is used as attenuator of the wave motion to protect the basin partially sheltered and as mooring quay with high stability. Such breakwater is a system of modular elements linked by flexible connections.The modular elements are floating structures made with reinforced concrete and are unsinkable thank to internal blocks realized in expanded polystyrene. The concrete is made by a mixture of pozzolan or blustfurnace cements, to be proper to the sea environment, and selected aggregates to guarantee a good quality. The polystyrene of the internal core is a closed cell and high-density polystyrene. The stainless steel reinforcements of the jetting are the result of the application of calculus models for analysis and of experimental tests.The decking is made of concrete superficially treated or of hardwood boards.The flexible connections in the head are made by neoprene blocks, crossed by steel ducts. The screwing up elements are in stainless steel.