Explanatory scenarios for the evolution of dinosaur anatomy are often couched in terms of competitive advantage, with those animals of superior design overtaking those that are slower and less efficient. Many, if not the majority, of the most striking structural modifications, however, are the product of competition and display between conspecifics during courtship. Among dinosaurs, ornithischians evolved an elaborate array of bony crests on the skull. These often constitute the most diagnostic portions of the entire skeleton. Traditionally these bony head accessories, typically sheathed and extended by horn, were viewed as the defensive weaponry of quiescent herbivores. More recently they have been reinterpreted on the basis of living analogs (such as horned ungulates) as secondary sexual characteristics employed primarily during intraspecific rivalry (see Norman's contribution to this volume).