Placoderms are the only known stem-group jawed vertebrates with jaws and their phylogenetic relationships have become central to the question of how gnathostomes evolved. Among placoderms, petalichthyids and "acanthothoracids" have taken a pivotal position in the debate on placoderm paraphyly, owing to their similarities with the jawless outgroups of jawed vertebrates. Nevertheless, their endocranial anatomy is still poorly known, preventing a thorough comparative analysis with the other early gnathostomes. In this thesis, I present the neurocranial anatomy of three exceptionally preserved placoderms, the petalichthyids Shearsbyaspis and Ellopetalichthys, and the "acanthothoracid" Kolymaspis. Using X-ray computed microtomography, I generated three-dimensional reconstructions of their endocranial surfaces, orbital walls, and cranial endocavity. The investigation of the endocranial anatomy of Shearsbyaspis revealed a combination of both derived and plesiomorphic characters never observed before in a placoderm, particularly in its orbital morphology and palatal architecture. Ellopetalichthys provided new evidence to help understanding the functional morphology and evolution of the inner ear and neck articulation in petalichthyids and early jawed vertebrates. Kolymaspis is revealed as an intermediate morphology between long- and short-nosed "acanthothoracids", suggesting that placoderms with similar orbitonasal anatomy might represent a highly specialized morphology rather than being plesiomorphic for jawed gnathostomes. With the aim of testing competing hypothesis on placoderm relationships, I performed different phylogenetic analyses on a new dataset comprising a wide range of early gnathostomes. Parsimony and Bayesian analyses resulted in competing hypotheses regarding placoderm monophyly and we compared the strengths and weaknesses of competing scenarios of placoderm relationships. Placoderms monophyly may indicate new scenarios for the early evolution of jawed vertebrates, implying a primitive architecture similar to those of early arthrodires, maxillate "placoderms" and bony fishes. Similarities between jawless fishes and certain placoderms may instead represent convergent adaptations.