The 180 species usually assigned to Polypodium appear to represent a polyphyletic assemblage of nearly exclusively New World species. Rooted to Old World genera, most Polypodium species occur in the continental upland tropics, with relatively little diversification on islands, and only the monophyletic P. vulgare complex found in temperate zones. Scale characters appear particularly significant for circumscribing apparently natural groups in this large genus. Glabrous and scaly-leaved species constitute separate lineages, with bicolorous, concolorous, and clathrate rhizome scales useful for further subdivisions. Other significant morphological features include complexity of vein anastamoses, numbers of soral rows, and patterns of rhizome branching. Separate and combined analyses of sequences from rbcL, rps4, and trnL-F yield insights for testing hypothesized subdivisions of Polypodium and determining interrelationships between Polypodium and other genera in the Polypodiaceae. Polypodium should be strictly applied to the glabrous-leaved species, whereas Pleopeltis appears to be the generic name that will be used for the scaly-leaved species. Some regularly included Polypodium species (e.g., P. triseriale and P. ptilorhizon) appear to have a tenuous connection to other members of the genus. Pecluma emerges consistently as an outgroup to the scaly polypods and not closely allied to the grammitids. Other close relatives may include Campyloneurum and Niphidium, although the precise position of these segregate genera has been difficult to determine. These analyses help provide targets for future biosystematic studies of tropical American genera and will aid in defining general patterns and processes of diversification among pteridophytes.

Key words: ferns, Polypodiaceae, Polypodium, pteridophytes, systematics