The vertical and horizontal patterns of the distribution of corals and coral reefs (to a depth of 90 m) are discussed in relation to the environmental factors: geomorphology of the bottom, available substrate, light, turbidity, sedimentation, water movement and temperature. There is a general pattern which is comparable to other well-developed Caribbean reefs. However, as in other areas variations are found, e.g. the depth and growth form of Acropora palmata will depend on the degree of exposure to water movement. There are strong correlations between the environmental variables and the occurrence of coral species and their growth form, the species composition of coral communities and the character of the coral reef. In some cases the relationship is not that obvious. The absence of Agaricia species at certain points along the coast of Aruba and the dominance of Sargassum on the deep bottom at some places along the windward coast of Curaçao is not yet explained. The relative importance of the different factors in an environmental setting is shown by a comparison of reef communities and reef habitats with a coral community of a muddy, shallow inland bay. The community of the bay consists, apart from the hardier coral species, of corals which are characteristic of the deep reef: S. cubensis Scolymia lacera, and Helioseris cucullata. These corals are adapted to sedimentation and low light intensities and are apparently able to withstand a temperature and salinity range much broader than that of their deep reef habitat. The paucity of corals and coral reef development around the islands of the Windward Group (deeper habitat included) can generally be explained by the morphology of the sea floor, the lack of suitable substrate and the effect of hurricanes. The exposed coasts of Saba and St. Eustatius, being virtually unexplored, may have richer coral growth. A new list of species of the Scleractinia of the Leeward and Windward groups, consisting of 57 species, is included.