This report describes experimental and numerical studies to investigate the the process of wave/breakwater interaction. Experimental tests were conducted in a one metre wave flume. The test structures were subject to monochromatic waves ranging in height and period. Various armour units and three types of core and filter materials were used. The test structures were instrumented with pressure transducers placed along the outer face of the structure and at the core/filter interface. Specially designed capacitance gauges were installed throughout the core to monitor the motion of the phreatic surface. Observations were made of wave runup and rundown, wave breaking and air entrainment in the structure. An additional set of experimental tests was taken to investigate the mechanism by which natural armouring of breakwaters gain their stability. Numerical studies were undertaken to provide an assessment of the performance of of an existing hybrid finite element method of characteristics model for analysisng the internal flow characteristics of a rubblemound breakwater.