Enantiornithes is a diverse group of Mesozoic birds, however little is understood about their interrelationships, and even their monophyly has been questioned. Repeated attempts to yield phylogenetic hypotheses at the species level have resulted in trees with low support that are largely inconsistent between matrices. These hypotheses consistently place Enantiornithes as sister group to Ornithuromorpha, together comprising the clade Ornithothoraces, which includes Neornithes. Because of their phylogenetic position, intermediate between Archaeopteryx and modern birds, as well as their success during the Cretaceous, enantiornithines are important for better understanding early avian evolution and the evolution of the anatomically modern bird. This large-scale study of enantiornithines has three main aspects; a taxonomic review in order to determine the validity of the over fifty named taxa, a morphological review, and a systematic study through a species level cladistic analysis of the group. Twenty-one species are here considered invalid, leaving 37 valid species in the literature. A cladistic analysis of 237 characters, including 42 novel enantiornithine characters, for 56 OTU's, 40 of which are enantiornithine, reveals 15,268 trees of 902 steps. The reduced strict consensus tree with three taxa removed is fairly resolved, although derived taxa still form a polytomy. Pengornis is resolved as the basal most known taxon, suggesting that size reduction in Mesozoic birds only occurred within the ornithothoracine clades. Interpretations of the current phylogenetic hypothesis suggest a Eurasian origin for Enantiornithes, a mid-Cretaceous global dispersal for Euenantiornithes, and suggest certain features such as the fan-shaped tail evolved independently within both ornithothoracine clades.