The Monocotylidae is a family of monogeneans parasitic exclusively on chondrichthyan fishes. Five of the 6 subfamilies (Calicotylinae, Monticelli, 1903, Decacotylinae Chisholm Wheeler & Beverley-Burton, 1995, Heterocotylinae Chisholm Wheeler & Beverley-Burton, 1995 and Merizocotylinae Johnston & Tiegs, 1922, Monocotylinae Taschenberg, 1879) are revised using additional material of 29 previously described species and type specimens of most nominal species. The Dasybatotreminae Bychowsky, 1957 is not revised because new material from this subfamily was not collected. I describe 7 new species of monocotylids from elasmobranchs caught off Heron Island and Tasmania, Australia including: Decacotyle tetrakordyle n. sp. from the gills of Pastinachus sephen; Heterocotyle capricornensis n. sp. from the gills of Himantura fai,· Empruthotrema tasmaniensis n. sp. from the nasal fossae of Myliobatis australis; Merizocotyle urolophi n. sp. from the nasal fossae of Urolophus paucimaculatus; Monocotyle corali n. sp. from the gills of Pastinachus sephen; Monocotyle jordani n. sp. from the gills of Myliobatis australis; and Monocotyle youngi n. sp. from the gills of Himanturafai. Papillicotyle Young, 1967 is synonymised with Decacotyle Young, 1967 because members of both genera have 1 central and 10 peripheral loculi and muscular structures on the dorsal surface of the haptor. Calicotyle rosinae Kusnetsova, 1970 is synonymised with C. macrocotyle Cordero, 1944, Neoheterocotyle trilobata Timofeeva, 1981 with N rhinobatis Pillai & Pillai, 1976 and Monocotyle trygoni Venkatanarsaiah & Kulkarni, 1980 with M spirophallus (Tripathi, 1959) Timofeeva, 1985. Calicotyle sjegi Kusnetsova, 1970, Heterocotyle elliptica Pillai & Pilai, 1976, H robusta (Johnston & Tiegs, 1922) Price, 1938 and N djiddensis Pillai & Pillai, 1976 are species inquirendae and Gymnocalicotyle inermis Woolcock, 1936 a species incertae sedis. I recognise 20 genera and 92 species of monocotylids as valid. Illustrations of all new species and the important taxonomic characters of described species are provided and I give keys to species for all the genera I revised. The morphology of the male copulatory organ of Heterocotyle granulatae Young, 1967, Neoheterocotyle rhinobatidis (Young, 1967) Chisholm, 1994 and Monocotyle spiremae Measures, Beverley-Burton & Williams, 1989 changes as the parasite matures and the ramifications of this observation for the identification of species of monocotylids are discussed. The morphology of the oncomiracidia of 9 species of monocotylids is described from live larvae and the number and distribution of the ciliated cells and sensilla in the oncomiracidia of 5 species of monocotylids are described using a silver-staining technique which is outlined fully. Using the distribution of the ciliated cells on the larvae, current subfamilial divisions defined by adult morphological characters are supported, except for Dasybatotreminae and Decacotylinae. The distribution and content of the gland cells in live larvae and of the anterior glands in live adults of representatives in several genera are outlined. The homology of the glands in the larvae with those in the anterior of the adult of Heterocotyle capricornensis n. sp. is reconciled. The complexity of the haptor is related to the habitat of the parasite on the host. Monocotylids that live in more turbulent environments such as the gills and dorsal skin have more complex haptors than do worms in less turbulent sites including the nasal fossae, cloaca, rectum, rectal gland and the body cavity. Components of the haptor differ even among congeners living on the "gills" of their hosts and these differences relate directly to the microhabitat among 3 species of Monocotyle Taschenberg, 1878. Studies on the development and distribution of Neoheterocotyle rhinobatidis, show that different structures on the haptor may be important for attachment to the host at different stages in the life of the parasite. The geographical distribution and host-specificity of the Monocotylidae also is discussed. Monocotylids may not be as host-specific as believed previously. I propose 3 criteria should be met to help resolve host-specificity in the Monocotylidae: accurate host identification, accurate parasite identification and a large database on host and parasite records. New collections of monocotylids indicate that none of these criteria is satisfied at present.