Published by the American Geophysical Union as part of the Antarctic Research Series, Volume 37.The spermatophyte and pteridophyte flora of Isla de los Estados, Territorio Nacional de la Tierra del Fuego, Antártida e Islas del Atlántico Sur, Departamento Ushuaia, Argentina, is presented in a systematic format. Detailed descriptions and analyses of the plant formations; associations and communities; ecological considerations; environmental factors; the island's geography, topography, and climate; and a brief historical outline of botanical exploration of the remote island accompany the taxonomic and nomenclatural accounts of the herbarium specimen collections resulting from Research Vessel (R/V) Hero cruise 71-5. The taxonomic catalog, entitled the Systematic and Annotated Synopsis of Vascular Plants, enumerates all genera, species, and infraspecific taxa of angiosperms and ferns that are positively known to be indigenous to Isla de los Estados, or that have been inadvertently introduced and naturalized (e.g., Taraxacum officinale). This systematic synopsis is based almost exclusively on the extensive botanical collections and field observations made on Isla de los Estados during October and November of 1971 by the collecting team of T.R. Dudley, R.N.P. Goodall, and G.E. Crow. These herbarium materials resulted from participation in the joint U.S.-Argentine botanical expedition, R/V Hero cruise 71-5 to Isla de los Estados, under the auspices of the Office of Polar Programs, National Science Foundation, Washington, D.C. The herbarium specimens collected at the very beginning of the expedition from Bahias (Bas.) Buen Suceso and Valentin at the extreme southern tip of Peninsula Mitre, Isla Grande, Territorio Nacional de la Tierra del Fuego, are incorporated into the systematic synopsis for purposes of phytogeographical comparision and analysis. These specimens and observations from Peninsula Mitre are of definite significance because Isla de los Estados is only about 40 km to the east, being separated by the turbulent Estrecho de Le Maire. A smaller collection of herbarium specimens from Provincia Magallanes, Chile, was accumulated before embarking aboard the R/V Hero cruise from Punta Arenas. These Chilean materials are also incorporated in the systematic synopsis, likewise for purposes of comparison, and because this collection, although small, reflects a portion of the accomplishments of the expedition.