Rubble-mound breakwaters are subject not only to wave action but, also, to other types of environmental loading, such as earthquakes. High seismic activity combined with soft foundation soil can lead to large settlements and even to failures of these structures. This paper reports on the experimental investigation undertaken to study the behaviour of rubble-mound breakwaters under seismic loading. Two models were built: the first one was designed to sit on a rigid bed, and the second on a yielding base of loose sand. Input earthquakes were of horizontal acceleration with increasing magnitude at consecutive tests. Measurements of hydrodynamic pressures and accelerations were taken. It was found that in general rubble-mound breakwaters resting on rigid bed are quite seismic resistant structures. Careful placement of the armour units provides extra strength to the armour layer as well as to the whole mound. The hydrodynamic forces on the faces of the break water, although small compared to the inertia effect on the structure, should be taken into account since they tend to modify the fundamental period of oscillation of the rubble-mound. Extra care should be taken during the design phase whenever rubble-mound breakwaters are to be placed on soft soil in areas of high seismicity.