Some of the biological problems presented by bracken,Pteridium aquilinum(L.) Kuhn, are posed. Its taxonomic position within the Pteridophyta and the delimitation of entities within the genus are discussed on the basis of morphological and cytological evidence. The geographical ranges of the various brackens world‐wide are described and mapped in outline, and emphasis placed on reviewing the natural ecological role of bracken in plant communities throughout the world. Further geographic areas where taxonomic investigation ofPteridiumis most needed are indicated, and evidence of the reproductive, dispersal, establishment, colonizing ability and vegetative persistence of bracken is reviewed. Its palaeobiological spread, with associated vegetational history, and the effects on this of anthropogenic influences—better known than are comparable details for any other pteridophyte—are detailed, and the present magnitude of the resulting bracken problem in Britain (and especially in upland Britain) indi