In this account of the lagoon systems of the Guinea Coast of West Africa, the distribution of lagoons is discussed in relation to the direction of the coast-line and the quarter of the dominant wind. Two types of lagoon are recognized according to the presence or absence of large rivers in the different regions. An aerial survey of the Dahomey and Western Nigerian lagoon systems has been made, and three types of beach accretion responsible for the seaward advance of the shore are described. It has been shown that the pattern of vegetation on lagoon deposits, being governed by the nature of the soil, can be used to indicate the position of the past beaches, sand ridges, spits and lagoons. An area to the west of Lagos has been studied by this method and a map prepared to show the successive advances of the shore.