A major concern to engineers engaged in the design of timber harbor structures is protection against marine borers. These pests can severely damage these structures in a relatively short time. Attack is concentrated on submerged timbers in the area between the mudline and the water surface. The intensity of attack is dependent on a number of environmental conditions. The most destructive and widely distributed borers are the Teredinidae and the Limnoria. Some forms of borers exist in all oceans. This paper describes the manner in which the borers destroy timber. It summarizes information gathered by various investigators on the conditions which have bearing on the presence of borers and the factors governing rate of destruction. Several methods of protecting timber structures from infestation are described and the costs compared.