This work contains a survey of the epilithic algal vegetations found on the coast of the Netherlands. The substrates on which these vegetations occur, are of recent origin; they all owe their existence to human activity (reinforced dikes, breakwaters, harbour works). The coast is divided in three geographical districts, viz. the “Waddenzee” district, the beach district, and the deltaic district. The latter is split in an euhalinc Scaldian subdistrict and a polyhaline estuarian subdistrict. The algal vegetation of the nearest natural rock coasts, viz. that of Heligoland and that of “le Boulonnais” in the English Channel are taken into consideration for comparison. In a following chapter the influence of the main factors operating in the environment is sketched. Then the delimitation of the supralittoral, the eulittoral and the sublittoral regions as well as the concepts basic and standard zonation are discussed. After that the communities are defined on account of their floristic composition, and combined into formations on account of the life form of their main constituents and their stratification. The formations themselves are classified according to their physiognomy and stratification and to the position they occupy in the zonation as well as in the succession. On the base of these criteria two systems of classification are possible, viz. one in which the community is regarded as consisting of several layers and in which emphasis is laid on the life form of the main consituents of the principal layer, and one in which the various layers are regarded as independent communities (socions). However, in this work the classification is not based on the socions, but on communities of the first-mentioned category. In the last chapter the various vegetation units are described by the aid of records containing qualitative as well as quantitative data. This enables us to compare them with vegetations occurring on other coasts. Most of the associations had not yet been recorded from the Netherlands. The study of the various species also led to the recognition of a new family of the Gladophorales, viz. the Codiolaceae. For two species so far included in Urospora the new combinations Codiolum hartzii (Rosenv.) and C. wormskoldii (Mert.) are made. At the end of each of the five chapters a partial summary is given.