The long bone histology of the “hypsilophodontid” dinosaurs Orodromeus makelai, Dryosaurus altus, and Tenontosaurus tilletti, was examined from perinate to largest available ontogenetic stages. These ontogenies were compared with each other, and to those of other dinosaurs, notably hadrosaurids such as Maiasaura and Hypacrosaurus. Orodromeus is a small dinosaur, and its more moderate growth trajectory is consistent with those generally observed for relatively small dinosaurs and other small ornithosuchians. Tenontosaurus achieves a relatively larger adult size, and its bone histology through ontogeny is similar to those of hadrosaurs, although reflective of slightly lower growth rates. Dryosaurus is thought to be a small dinosaur, but in its largest recognized ontogenetic stages it does not display the “adult” histological features of other ornithopod adults, although it does suggest growth rates comparable to those of larger ornithopods, including hadrosaurs. Determinate growth is observed in Orodromeus, Tenontosaurus, and hadrosaurs, so we infer that the adult stage of Dryosaurus has not yet been recognized. Dinosaurs continued to grow slowly after reaching adult size, so the largest known specimens of most taxa are not maxima.