This revision is the result of a study of the genotype species, mostly of type or topotype specimens, of nearly every described scleractinian genus. The classification proposed rests primarily on the structure of the septa, but other skeletal structures as well as the soft parts are utilized and considered. The order Scleractinia is divided into five suborders: Astrocoeniida, Fungiida, Faviida, Caryophylliida, and Dendrophylliida. The first is quite distinct from the others and includes corals with septa composed of relatively few trabeculae. The other four include corals in which the septa consist of a relatively large number of trabeculae. The Fungiida is marked by the fundamentally fenestrate arrangement of the trabeculae, whereas the arrangement is laminar in the Faviida, Caryophylliida, and Dendrophylliida. In the Faviida the septal margins are dentate; in the Caryophylliida they are smooth; and in the Dendrophylliida some septa may become secondarily lacerate or dentate. The subdivision of the suborders into lesser categories is based upon the modification of septal structure and other skeletal elements, mode of colony - formation, form of the corallum, and phylogeny of the groups. The systematic classification is prefaced by an historical résumé of previous investigations of the Scleractinia and a brief discussion of the anatomy and morphology of the polyps and skeletal structures. The several different modes of sexual and asexual reproduction and increase are carefully analyzed because of their relation to growth form and from this is developed a discussion of morphogenesis of the corallum. Considerable attention is paid to the ecology of recent corals—much is known concerning the reef-building forms, but certain aspects of the ecology of the ahermatypic or non-reef-builders are here extensively considered for the first time. The distribution of fossil and recent scleractinian faunas is broadly analyzed and some suggestions concerning the evolution of the order are made. A classified list of over a thousand titles dealing with all aspects of the Scleractinia and fifty-one plates illustrating nearly three-fourths of the approximately 500 genera recognized conclude the work.