This book comprises the redescrption of an excellent specimen of Tenontosaurus tilletti. Previous phylogenetic placement within the Iguanodontia is questioned based on the lack of character affinity with the defining synapomorphies of the clade. The clades Hypsilophodontidae (=Hypsilophodontia) and Iguanodontia are both deemed to be in critical need of refinement to account for recent discoveries and re-classifications of certain euornithopods if ornithopod relationships are to be resolved. Several synapomorphies are out-dated and deemed redundant in favour of a more quantifiable approach. Phylogenetic studies must be approached from a multidisciplinary perspective; integration of tectonostratigraphical, ontogenetic, palaeoecological, and biomechanical data with sets of well-defined primary homologies are essential in increasing phylogenetic resolution and generating stratigraphically-feasible ancestor-descendant relationships. Furthermore, given the quantity of material assigned to Tenontosaurus, and it's temporal and spatial range, this material requires closer inspection to resolve intra-generic variations.