Two new and twelve little known species of Phyllobothrium or of allied genera, mainly from elasmobranchs and teleosts caught off the British Isles, have been investigated. As some of these could not be identified to species and the others presented difficulties, material from various other sources was obtained and examined for comparison. In addition, the literature on about 100 species allocated to Phyllobothrium was consulted and brought together. A critical appraisal of this literature is given and the genus is revised for the first time. Information on most of the species was found to be inadequate to provide a key and, therefore, a host-parasite list was compiled. As only about fifty species of Phyllobothrium sensu lato have been found in about 100 of the 3000 species of elasmobranchs known to exist, it is estimated that a very large number of Phyllobothrium spp. remains to be discovered and described. The possible significance of this fact is discussed. Brief descriptions of the most well-known species of Phyllobothrium are given and reasons against listing synonyms for these are emphasized. Of the 100 species already allocated to the genus only twenty-two are accepted at present; further studies may show that only three of these, viz. P. lactuca, P. dagnallium and P. serratum, show the typical features of the genus, as originally described. Fourteen of the twenty-two may, eventually, be placed in Anthocephalum Linton, 1890, if this genus is revived; the erection of a new genus or genera may be necessary for the remainder. P. britannicum sp.nov., from Raja montagui, is provisionally placed near P. lactuca but the bothridia are only slightly bifid, their margins are not so folded and the species is euapolytic. P. minutum sp.nov. from R. fullonica closely resembles P. auricularia, P. foliatum and P. loculatum and may, eventually, fall as a synonym of one of these; at present they are all little-known forms. Reasons are given for provisionally retaining Crossobothrium and Monorygma, with about ten species in each. It is suggested that three species originally placed in Phyllobothrium may be allied to Sphaerobothrium lubeti Euzet, 1959. An examination of the type material of P. ketae Canavan, 1928, previously regarded as unique or as a neotenic form, has shown that the original description was partly based on a pseudophyllidean and possibly on the larvae of P. caudatum. A number of larvae of Phyllobothrium in invertebrates, teleosts and marine mammals, fourteen little-known species of the genus and invalid members are discussed. A detailed discussion is given of the ecology, host specificity and attachment of Phyllobothrium and allied genera to the gut mucosa of elasmobranchs. In a general discussion brief comments are made on life-history, the identification and classification of Phyllobothrium, self-fertilization and on 'segmentation'. Almost 200 references are cited.