The objectives of the Port Sines Investigation Panel were to collect perishable data on and to identify causes of the failure on February 26, 1978, of the rubble mound breakwater at Port Sines, on the Atlantic Coast of Portugal. This report is a result of that investigation. The structure is the largest of its kind in such an exposed environment, and is situated in a previously untried combination of unusually deep water and high-energy marine setting. Five simplified failure possibilities are evaluated: (1) that design criteria were exceeded by the February 1978 storm: (2) that the breakwater construction was faulty; (3) that the materials used for construction were substandard; (4) that the procedures followed during the design were incomplete or incorrect for this specific set of environmental conditions; (5) that the design was inadequate. The purpose of this investigation was not to find a crucial error or omission but to report on all possible problems, omissions, and errors that could have contributed to the failure. A description of the breakwater project is given, along with details of the design and construction of the project. The physical setting and the storm of February 26, 1978 are also described. This pdf also contains a number of photographs and drawings of the failure from other sources than the Panel report.