The purposes of this enumeration are to summarize and clarify our knowledge of the occurrence of species of ferns and fern-allies in Illinois; to account for binomials accredited to Illinois during the past one hundred and fifty years; and to cite specimens to authenticate the county distribution. The present study includes 64 species in 27 genera and 10 families. In order to make it more useful, keys and diagnostic descriptions for families, genera, and species are provided. In the treatment of species, the valid name with its place of publication, and basonym, as well as a brief synonymy of names that have been applied by Illinois authors, have been included. In addition, bibliographical references, type locality, habitats of species in Illinois, and a statement of the total known geographical distribut on are given briefly, as well as local distribution, so far as it is known, in Illinois by counties. The method of indicating general distribution of species has been adapted from Rydberg's Flora of the Prairies and Plains. A number of articles, monographs, and books have been consulted frequently during the preparation of this paper. These are included in the bibliography. The following books have been of particular use: M. Broun, Index to North American Ferns, W. R. Maxon, Pteridophyta in the second edition of Britton & Brown's Illustrated Flora, J. K. Small, Ferns of the Southeastern States, R. M. Tryon, N. C. Fassett, et al., Ferns and Fern-allies of Wisconsin, and E. T. Wherry, Guide to Eastern Ferns. Cited specimens, unless otherwise indicated, are in the herbarium of the University of Illinois. Other material has been studied and some of it cited, principally from Chicago Natural History Museum (FM), Illinois Natural History Survey (NHS), Illinois State Museum (SM), Missouri Botan;cal Garden (MBG), and Peoria Academy of Science (PAS). Thanks are due V. H. Chase, R. A. Evers, G. D. Fuller, J. M. Greenman, P. C. Standley, J. A. Steyermark, and L. R. Tehon for their kind permission to incorporate data from this material into the present study. I should also make grateful acknowledgment to Professor C. A. Weatherby of the Gray Herbarium for his many helpful suggestions, and to Dr. E. W. Fell of Rockford, Illinois, for information concerning the fern-flora of his district.