The present paper is a critical review of data and hypotheses on the head segmental composition in chelicerates and in extinct non-mandibulate arthropods. It successively takes into account data from morphology and embryology, from the structure of the nervous system, from palaeontology and from developmental genetics. Discussion focuses on possible homologies between the head segments and appendages in arachnomorphs and those in mandibulates. The comparative anatomical and ontogenetic data, especially those concerning the central nervous system, its connections with the stomatogastric system, and head innervation, show many similarities between the head organization of chelicerates and that of mandibulates, and lead to conclusions that contradict some of the hypotheses deduced from recent studies on developmental biology, but favour more traditional views. In particular they support the presence of a deutocerebral segment in the head region of the ground pattern of arthropods and its loss in all extant chelicerates. They also support the homology of the cheliceral ganglia with the tritocerebral ganglia of mandibulates. The possible existence of a precheliceral segment and of a presegmental acron remains open to question.