This monograph is a synthesis of the diversity, systematics, host associations, and biogeography of the Diphyllidea. A thorough review of the literature resulted in the transfer of Diagonobothrium into Diphyllidea as a genus inquirendum. The genus Yogeshwaria was transferred to Diphyllidea as a synonym of Echinobothrium. Its only species, E. nagabhushani n. comb., is considered to be a species inquirenda. New collections resulted in the description of a new species of Echinobothrium. Type and/or voucher specimens for 32 of 36 valid diphyllidean species (including the new species) were examined using light microscopy and scanning electron microscopy. All 32 species examined were redescribed and figured.