The marine calanoid copepod family Euchaetidae and its two genera, Euchaeta and Paraeuchaeta, are redefined. Fourteen species of Euchaeta and 61 species of Paraeuchaeta, including 13 new species, are described and their geographic ranges defined from specimens found in midwater trawl and plankton net samples collected throughout the Atlantic, Pacific, and Indian oceans. The species of Euchaeta are classified into three species groups - marina, concinna, and acuta groups - and an independent species, E spinosa; those of Paraeuchaeta were classified into six species groups malayensis, pavlovskii, norvegica, glacialis, hebes, and Antarctica groups - and three independent species - P. biloba, P. bisinuata, and P. grandiremis. Each of the nine species groups is defined with detailed descriptions of its representative species and each of the four independent species that could not be grouped is also described in detail. Phylogenetic relationships among the species groups and independent species are discussed. Keys are presented for identification of the species groups in each genus and the species in each species group containing three or more species. The geographic distribution of the species groups and common species is discussed.