This important group of ornithischian dinosaurs has a history that extends from the Early Jurassic until the close of the Cretaceous Period. The group as a whole can logically be subdivided into three quite distinct lineages: the heterodontosaurs, the hypsilophodonts, and the iguanodonts. I shall discuss the attributes of these groups separately, so that direct comparisons can be drawn by the reader of the way in which ornithopods as a whole evolved and diversified during the latter part of the Mesozoic. However, before going on to deal with these individual groups of ornithopods in a little more detail, I shall briefly outline their history and degree of relatedness.