This paper provides a compilation of the genus Metaxonchium. The variations observed in its most interesting morphological traits (body size and shape, cuticle, lip region and amphid, stoma and feeding apparatus, pharynx and pharyngo-intestinal junction, female genital system, male genital system and its accessory elements, and caudal region), which are relevant to elucidating its taxonomy and evolutionary relationships, are described and discussed. The taxonomy of the genus is revised, including an emended diagnosis, list of species (30 valid and one inquirenda) and their synonyms, key to their identification, and a compendium of morphometrics of all the previously known species and their populations. New molecular data (18S, 28S rDNA sequences) are presented for Metaxonchium giennense. The evolutionary relationships of the genus are discussed with an integrative approach, combining morphological and molecular information. The presence of a conspicuous pars refringens vaginae is interpreted as the most remarkable synapomorphy of the genus and as an argument for its monophyly. Molecular analyses, however, do not totally corroborate this monophyly, but they are based on a very limited number of species. The monophyly of the subfamily Belondirinae is highly supported, as well the existence of two main subclades within it. The polyphyly of Belondiridae is once more confirmed. The rank of Spiculaxonchium is raised from subgeneric to generic level.