This paper reports on a laboratory investigation conducted at the United States Army Engineer Waterways experiment station, Vickburg, Mississippi, to determine criteria for the design and construction of rubble mound breakwaters. Small-scale breakwater sections are hand-constructed in a concrete wave flume 119 ft long, 5ft wide, 4 ft deep, and subjected to mechanically generated waves to determine the stability of the armor units. A general stability equation has been derived and is being used to guide the experimental program and correlate the test data. From the test data obtained important unknown functions in the general stability equation have been determined for selected breakwater and test-wave conditions, and a new breakwater stability formula has been obtained. In conjunction with the stability tests, wave run-up data are obtained for each breakwater section and wave condition tested. Also, measurements are obtained that enable the thickness and porosity of cover layers composed of different types of armor units to be determined. The new stability formula and the experimental data obtained so far have provided essential information for an improved method of designing rubble-mound breakwaters with protective cover layers composed of quarry-stone and tetrapod armor units. Tests in progress (1959) to obtain experimental data for other special shapes of cast concrete armor units (cubes, tetrahedrons, and tribars) should increase considerably the accuracy of rubble-mound breakwater design.