An examination of morphological characters of the Liljeborgiidae reveals that these uncalceolated gammaromorphic amphipods share a number of putative plesiomorphic characters with the Melphidippoidea and the Oedicerotoidea. The Liljeborgiidae are subdivided in two subfamilies: the Idunellinae subfam. nov. for Idunella G.O. Sars, 1894 sensu lato (including Listriella J.L. Barnard, 1954) and Sextonia Chevreux, 1920, and the Liljeborgiinae Stebbbbing, 1899 for Liljeborgia Bate, 1862 sensu lato (including Isipingus J.L. Barnard & Karaman, 1987). A list of all known Liljeborgiidae is given. Idunella aeqvicornis (G.O. Sars, 1877) (the type species of the genus), Idunella picta (Norman, 1889) (a species partly agreeing with the characters of the genus Listriella, which is provisionally treated as a junior synonym of Idunella) and Sextonia longirostris Chevreux 1920 (the most plesiomorphic Liljeborgiidae known to date) are re-described. Idunella proves to be morphologically very diverse but the splitting of the genus appears premature. Three groups are recognized within the genus Liljeborgia. Groups 1 and 2, which are cosmopolitan, speciose and well represented in Europe, are considered as separate subgenera: Liljeborgia Bate, 1862 and Lilljeborgiella Schellenberg, 1931. Group 3, which was not available for study, includes a few little-known Northwestern Pacific species and the very insufficiently described South African L. epistomata K.H. Barnard, 1932 (= Isipingus epistomatus). The North East Atlantic and Mediterranean species of the subgenus Liljeborgia are revised and the following taxa are separately treated: Liljeborgia brevicornis (Bruzelius, 1859), L. dellavallei Stebbbbing, 1906, L. inermis Chevreux, 1920, L. kinahani (Bate, 1862), L. macronyx G.O. Sars, 1894, L. mixta Schellenberg, 1925, L. pallida (Bate, 1857), L. psaltrica Krapp-Schickckel, 1975 and Liljeborgia sp. 4. These species include the type species of the genus Liljeborgia: L. pallida. Remaining difficulties to separate L. brevicornis, L. dellavallei, L. kinahani, L. mixta, L. pallida and Liljeborgia sp. 4 are pointed out and the validity of some of these taxa remains questionable. The rare L. inermis, which was previously only known from Mauritania and the Cape Verde Islands, is now recorded from South Portugal. The close similarities between the Scandinavian upper-bathyal species Liljeborgia macronyx and the lower-bathyal Antarctic species L. cnephatis d’Udekem d’Acoz, 2008 (both eyeless species) are pointed out, and it is suggested that the ancestors of L. macronyx could have lived in the Southern Ocean. In the subgenus Lilljeborgiella, Liljeborgia caliginis d’Udekem d’Acoz & Vader, 2009 is considered as a probable synonym of L. charybdis d’Udekem d’Acoz & Vader, 2009. An identification key and a checklist are provided for all known Mediterranean and North East Liljeborgiidae. It is pointed out that the names with ‘qv’ introduced by G.O. Sars, like Idunella aeqvicornis (G.O. Sars, 1877) have to retain their original spelling and that the ‘qv’ may not be converted into ‘qu’ as it is usually done in literature. In an appendix to the present paper, two melphidippoid families, cited in literature but previously nomenclaturally invalid, are herein valided: the Cheirocratidae fam. nov. and the Hornelliidae fam. nov.