Being engaged during several years with a revision of the grasses preserved in the Rijks-Herbarium at the University of Leyden, my attention was called to the group of the Stipeae, and especially to the very difficult genus of Aristida. After an exhaustive study of the literature, I thought it desirable to have a monograph of this genus, containing extensive keys for the determination of all the species hitherto known, and I resolved to prepare such a work. It has been my good fortune that I had at my disposal not only the valuable collections of the Rijks-Herbarium, but that by the courtesy of the directors of the great herbaria in Europe and in America, I could study many thousands of specimens, among them authentic specimens and types. So several years elapsed before the revision was finished. Before I am going to publish my work, it seemed desirable to prepare a preliminary paper on the subject, dealing with the literature studied and the results of the critical examination of the types, moreover the new species found in herbaria are included in this paper. To find easily the original description and the type specimen, I give in alphabetical order all the species and varieties hitherto described, no matter if they are accepted in my monograph as valid or not.