Most texts on the subject of dam construction state flatly that under no circumstances should an earth dam be permitted to overtop, and that spillway capacity should be such as to ensure this is achieved. The problems of flood forecasting are outlined and a survey of failures and accidents caused by overtopping presented. Information from this survey highlights weaknesses both in safety legislation and in operational procedures. These are presented, together with recommendations relevant to the design and construction of embankment dams. The properties of clays which are relevant to the problem of erosion are investigated by literature survey and laboratory research and the results presented. The mechanism and degree of erosion are detailed in an attempt to quantify the amount of erosion likely to occur for any particular clay. Recommendations are made with respect to areas where more research would be beneficial.