Concrete has clearly emerged as the most economical and durable material for the building of the vast majority of marine structures. Reinforced concrete too has overcome the technological problems making it a suitable material for the construction of advanced marine structures such as offshore drilling platforms, superspan bridges and undersea tunnels. As the world becomes increasingly ocean-oriented for energy and other resources it is predicted that construction activities during the 21st century will be dominated by concrete sea structures. The performance of concrete in the marine environment is presented here in a logical manner giving state-of-the-art reviews of the nature of the marine environment, the composition and properties of concrete, history of concrete performance in seawater, major causes of deterioration of concrete in the marine environment, selection of materials and mix proportioning for durable concrete, recommended concrete practice and repair of deteriorated marine structures. It is of value to any design or construction engineer responsible for marine structures.