External and internal structures of larvae of Elateriformia were studied and interpreted phylogenetically. Detailed descriptions of the head of representatives of the superfamilies Dryopoidea (Lanternarius sp.; Heteroceridae), Elateroidea (Melanotus sp.; Elateridae), and Cantharoidea (Phosphaena sp.; Lampyridae) are presented. Internal structures of larvae of Eulichadidae, Callirhipidae, Psephenidae, Dryopidae, Limnichidae, Artematopidae, Drilidae, and Homalisidae were examined for the first time. Loss of the mandibular mola is a common derived feature of Elateriformia. Prognathism, the presence of a broad, sclerotized gula, and an abdominal apex which is formed by tergite IX are considered as synapomorphies of Dryopoidea + Elateroidea + Cantharoidea. The presence of a characteristic maxillolabial complex and vertically arranged extrinsic maxillary muscles strongly support a monophylum comprising Dryopoidea (excluding Eulichadidae and Ptilodactylidae partim), Elateroidea, and Cantharoidea. As currently defined, Ptilodactylidae are paraphyletic. A monophylum comprising Eulichadidae and Ptilodactylidae (excluding Araeopidius and Cladotominae) is supported by the presence of a rigid, articulated mandibular process and paired lobes of segment X, which border the anus laterally. The monophyly of Elateroidea, Cantharoidea, and Dryopoidea (excluding Callirhipidae, Eulichadidae, and Ptilodactylidae part.) is suggested by apomorphic features of the tentorium and by the origin of the vertical maxillary muscles from the strongly modified posterior tentorial arm. The monophyly of Dryopoidea (excluding Callirhipidae, Eulichadidae, and Ptilodactylidae part.) remains open to question. A sister-group relationship between the dryopoid families Limnichidae and Heteroceridae is indicated by a specialized vestiture of setae, a hyaline antennal appendage, and a sclerotized, cranial tentorial connecting bar. The monophyly of Araeopidius, Cladotominae, Chelonariidae, Psephenidae, Dryopidae, Lutrochidae, and Elmidae is suggested by the presence of a ventrally hinged operculum of segment IX. The presence of plumose setae on the mouthparts is a possible synapomorphy of Chelonariidae, Psephenidae, Lutrochidae, and Elmidae. The presence of three retractable anal gills is considered as a significant synapomorphy of the latter three families. The monophyly of Elateroidea + Cantharoidea is indicated by partial reduction and immobilization of the labrum, loss of labral muscles (Brachypsectral), the presence of a strongly developed M. tentoriohypopharyngalis lateralis, and the presence of a setose pre-oral filter (groundplan). Complete fusion of clypeus and labrum is considered as a possible synapomorphy of the elateroid families (excluding Artematopidae) on the one hand, and of Cantharoidea (excluding Brachypsectridae) on the other. Highly aberrant, plate-like mandibles with restricted mobility and longitudinal sclerotizations of the presternum are considered as synapomorphies of Cerophytidae, Throscidae, and Eucnemidae. An unusually small and wedge-shaped head capsule, antennae inserted on conspicuous elevations, and a lightly sclerotized, grub-like body are probably synapomorphies of the latter two families. Strongly sclerotized thoracic and abdominal segments with the pleura largely or completely concealed may point towards a closer relationship between Cebrionidae and the elaterid subgroup Elaterinae. The monophyly of Cantharoidea is suggested by the presence of a longitudinal mandibular groove (groundplan). The direct attachment of the vertical maxillary muscles to the posthypopharynx is probably a synapomorphic feature of Homalisidae, Lycidae, Drilidae, Phengodidae (?), and Lampyridae. The monophyly of Homalisidae, Phengodidae, and Lampyridae is strongly supported by the presence of abdominal luminous organs. Fifty-four equally parsimonious PAUP-generated trees required 37 steps whereas 39 steps were needed in the hand-made cladogram. A monophylum suggested by PAUP, comprising Artematopidae, Cebrionidae, Elateridae, and Cantharoidae is in contrast to the results of the mental phylogenetic analysis.