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This thesis presents the results of a laboratory study of beach erosion. The study compares the erosion and accretion on an unprotected dune beach with a beach protected with a rubble mound sea wall. There has been considerable debate in the coastal engineering community about the effects of sea walls on beaches. Accurate, controlled experiments are required to compare how sea walls and natural beaches interact with waves. This experiment was conducted in the J. Robert Gunther Family Ocean Wave Facility in the Parsons Laboratory at MIT. The experiment used a 4 meter wide by 11 meter long section of the basin. The beach is split in two sections, one with an unprotected dune and the other with a 1:1.5 sloping rubble mound sea wall. A probe that uses reflected light to measure the sand or water surface is used to measure the sand elevation. This is mounted on a square X-Y plotter capable of moving the probe anywhere within a 4 meter by 4 meter area. Using the probe on the X-Y plotter, the surface could be mapped out as a series of X, Y and Z values. The beaches were subjected to various normally incident spectral and monochromatic waves. The experiment was run iteratively, alternating waves and beac