Taylor, E. L., T. N. Taylor (Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Ave., University of Kansas, Lawrence, KS 66045), H. Kerp (Forschungsstelle für Paläobotanik, Westfälische Wilhelms-Universität Münster, Hindenburgplatz 57, D-48143 Münster, Germany), and E.J. Hermsen (Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Ave., University of Kansas, Lawrence, KS 66045). J. Torrey Bot. Soc. 133: 62–82. 2006.—Mesozoic seed ferns represent a grade of gymnospermous plants whose affinities remain problematic. The three major orders recognized today include the Caytoniales (Triassic-Cretaceous), Peltaspermales (Carboniferous-Triassic) and Corystospermales (Triassic-Cretaceous). A number of genera described from Mesozoic rocks have also been included broadly in the Mesozoic seed ferns, but their frequency, distribution, and affinities render their assignment to specific orders equivocal. The morphotypes in the three principal orders have been important in phylogenetic analyses of seed plants and have been implicated as angiosperm progenitors at various times in the past. All three groups were originally described only from compression/impression fossils, but anatomically preserved corystosperms are now known from Argentina and Antarctica. Since their original description, the geographic and stratigraphic ranges of all three major groups have been expanded, and they are now known from both the northern and southern hemispheres. An additional order of Mesozoic seed ferns, the Petriellales, has been described from the Triassic of Antarctica. This paper will summarize our current knowledge of the Mesozoic seed ferns and comment on the phylogenetic position of several orders, focusing especially on permineralized and compressed corystosperms from the Triassic of Antarctica. Recent studies of well-preserved material from the central Transantarctic Mountains have provided information about the three-dimensional morphology and anatomy of pollen organs and ovulate cupules, as well as the first evidence of the attachment of reproductive organs to the parent plant. These discoveries offer new information that can be used in phylogenetic analyses to provide increased resolution of seed plant relationships.