This systematic treatment of the ferns and allied plants provides a modern classification of the Pteridophyta based on an assess- ment of the wealth of new data published during the last few decades as well as on our own research. The accounts of the gen- era include systematics, ecology, geography, spores and cytology and often other aspects of their biology. The scope of the work has involved all genera and the Old World species of those gen- era represented in America. For a few American genera, that are especially complex in the Old World, it has been necessary to limit their treatment to the American representatives and this is specifically indicated in those cases. The eight American genera that do not occur in the tropics are treated in less detail. They are Camptosorus, Gymnocarpium, Matteuccia and Onoclea of North America, Arthropteris and Thyrsopteris of the Juan Fernandez Islands, Pleurosorus of southern South America, and the amphi- tropical Cryptogramma. The complete generic nomenclature is included, except in a few specified cases where taxonomic complexities of the Old World elements have not been resolved. The synonyms and ac- cepted names of subgeneric and sectional taxa are included only when pertinent to the generic nomenclature or to an infrageneric classification. Only the name of a publishing author is cited. There is no bibliographic purpose served by including the name of a person who was the source of, but did not validly publish, a name or epithet.