The Megacalanidae were revised based on new and archived material. Taxonomic confusion that has existed in the family is discussed and a method is suggested for stabilising names. A detailed examination of the morphology of this family, using the light microscope, has added further useful characters that distinguish genera and species. The added, hitherto undescribed species include character states incompatible with aspects of previous generic definitions (e.g. presence or absence of setae on the maxillule coxal endite). Nevertheless, the cladistic and molecular analyses confirmed that there are at least four monophyletic clades mostly with high bootstrap support. These clades represent already defined genera, one of which [Elenacalanus nom. nov. (nomen novum)] replaces the preoccupied name Heterocalanus Wolfenden, 1906. Four previously described species have been re-assigned to Elenacalanus in new combinations: E. princeps (Brady, 1883), E. eltaninae (Björnberg, 1968), E. sverdrupi (Johnson, 1958) and E. inflatus (Björnberg, 1968). Eleven new species are described: three Megacalanus, one Bradycalanus, six Bathycalanus, and one Elenacalanus nom. nov. Bradycalanus pseudotypicus enormis Björnberg, 1968 has been raised to species status based on genetic data although it can be only be distinguished morphologically from Br. typicus by its large size. All four genera are differentially diagnosed and keys are provided to the genera and species. We confirm that all male right antennules are geniculate in the Megacalanidae. Thirteen males are known. Of these males, eight are newly described (M. frosti n. sp., M. ericae n. sp., M. ohmani n. sp., Bathycalanus bradyi (Wolfenden, 1905a), Ba. dentatus n. sp., Ba. milleri n. sp., Ba. unicornis Björnberg, 1968, and Elenacalanus tageae n. sp.). We cannot be absolutely certain that the correct males have been assigned to the appropriate female so our decisions await testing with new data. The cladistic analysis provides the first morphology-based phylogeny. This scheme served as a working hypothesis which was tested and corroborated using the newly gathered molecular data. Vertical and horizontal distributions are summarised.