In recent years, dinosaurs have captured the attention of the public at an unprecedented scale. At the heart of this resurgence in interest is an increased level of research activity, much of which is innovative within the field of paleontology. Whereas earlier studies emphasized basic morphology and taxonomy, modern studies develop our understanding of what dinosaurs were like as living animals. More than ever before we understand how their bodies worked, how they behaved, how they interacted with their surroundings and with each other, and how they changed over time. Nevertheless, these studies still rely on certain basic building blocks, including knowledge of anatomy and taxonomic relationships.

One of the aspects that we understand better than before is ontogenetic, sexual, and individual variation within a species. This helps us to evaluate our understanding of dinosaurs as biological species. Studies in progress are giving us a good understanding of all forms of variability for one or more species of each of the major groups of dinosaurs, including theropods (Chapters 6, 7), hypsilophodonts (Horner and Weishampel 1988), hadrosaurs (Horner and Makela 1979; Dilkes 1988; Horner and Weishampel 1988), iguanodonts (Norman 1987), protoceratopsians (Brown and Schlaikjer 1940; Kurzanov 1972; Dodson 1975b; Maryańska and Osmólska 1975), ceratopsians (Chapters 16, 18; studies in progress on Centrosaurus and Pachyrhinosaurus), and ankylosaurs (Maryańska 1971; studies in progress on new Pinacosaurus material from China).