This book is a comprehensive revision of the suborder Productoidea, a large group of Upper Paleozoic brachiopods characterized by open tubular spines. All the genera described in the extensive world literature are included, and the original descriptions are supplemented, whenever possible, by additional information obtained from examination of specimens in collections available to the authors. The stratigraphic range and geographic distribution of the Productoidea are reviewed. They range from Lower Devonian (Onondaga) to Upper Permian, a span of about 125 million years. In an extensive discussion of the external and internal morphology of the productoids new information concerning the shell interior and new interpretations of other anatomical features are presented. The terminology applied to the Productoidea is listed alphabetically. Descriptions and illustrations are given of each term employed. Life habits of the productoids are described in detail. Spines are thought to have played an important role and to have served either to anchor or attach the shell, or to steady it and keep it from being buried. The origin of the Productoidea is still problematical. It is shown, however, that they could not have been derived from the chonetoids or from the stropheodontoids; an early strophomenoid Leptaenisca with similar life habits and internal structure may have been an ancestral form. The history of productoid classification is traced from the early attempts by de Koninck and de Verneuil, which are based wholly on exterior form and ornament, to the later classifications in which interior characters and life habits play a role. The classification proposed here makes use of exterior characters and internal details, but all features of the productoid shell are considered. Emphasis is placed on the cardinal process in defining families, and nine types of process are described. Two major lines of descent are traced, and the two groups of genera are separated into the superfamilies Strophalosiacea n. superfam. and Productacea. The Strophalosiacea are cemented throughout life by beak or clasping spines and have inter-areas which may be present in both valves or in the pedicle valve only, or rarely are absent; the pedicle valve is conical, and the brachial valve opercular in some families, and the cardinal process is usually prominent, bilobed or trilobed. The Productacea are seldom cemented except in the youngest stages and when adult usually live loose on the sea bottom. They are generally attached by ringlike spines in early growth stages. Interareas, teeth, and sockets are only weakly developed in primitive forms, and the cardinal process is bilobate, trilobate, or quadrilobate. New families of the Strophalosiacea are: Aulostegidae, Sinuatellidae, Spyridiophoridae, Teguliferinidae, and Tschernyschewiidae. New families of the Productacea are: Buxtoniidae, Gigantoproductidae, Institinidae, Leioproductidae, Overtoniidae. Twenty-nine subfamilies are also proposed. It is only natural that careful restudy of the Productoidea would reveal many new genera, especially when the collections studied are so rich in specimens showing internal characters as those of the U. S. National Museum. A total of 167 valid genera are described and illustrated. In addition 5 doubtful names, 3 without description, are considered. Of the 167 genera 66 are new. New genera of the Strophalosiacea are: Chonosteges, Coscinarina, Costellaria, Craspedalosia, Crossalosia, Dasyalosia, Devonalosia, Echinosteges, Edriosteges, Lialosia, Rhamnaria, Sphenalosia, Sphenosteges, Xenosteges. The new genera of Productacea are: Acanthatia, Acanthoplecta, Alifera, Auloprotonia, Bathymyonia, Calliprotonia, Cinctifera, Costiferina, Costispinifera, Echinaria, Echinauris, Ericiatia, Fimbriaria, Flexaria, Fluctuaria, Galeatella, Geniculifera, Globietta, Grandaurispina, Hystriculina, Inflatia, Institifera, Institina, Laminatia, Levitusia, Marginatia, Megousia, Monticulifera, Orbinaria, Ovatia, Paucispinifera, Peniculauris, Pulchratia, Quadratia, Retaria, Reticulatia, Rhytiophora, Rudinia Rugatia, Rugauris, Rugicostella, Rugivestis, Semicostella, Sentosia, Septarinia, Squamaria, Stegacanthia, Stipulina, Titanaria, Tubaria, Tyloplecta, Undaria . An extensive bibliography of literature pertaining to the productoids completes the book. Nearly all the genera are illustrated by specimens including interiors, but a few are known only from the original illustrations which are reproduced.