The term dinosaur usually refers to reptiles, many of them large, that dominated the Earth during the Mesozoic era. The earliest dinosaurs lived during the Triassic period. Dinosaurs diversified into numerous forms during the Jurassic period and Cretaceous period, perhaps because one of the mass extinctions, which occurred at the end of the Triassic period, removed competition from other vertebrate groups. The dinosaurs themselves became extinct at the end of the Cretaceous period.

Modern phylogeny classifies organisms based on evolutionary divergence. Since birds and mammals diverged from the group of vertebrates usually called reptiles, there is no such thing as reptiles unless one includes birds and mammals in the group. Dinosaurs had many characteristics of skin, teeth, and bones that would remind observers of the reptiles with which they are familiar. In fact, since birds evolved from dinosaurs, the cladistic approach would indicate that the dinosaurs still exist: They are birds. Therefore the dinosaurs that most people think of are more properly called non-avian dinosaurs. Although dinosaurs are truly ancient, the therapsid (mammallike) reptile lineage had diverged before the dinosaur lineage, and true mammals already existed by the time the first dinosaurs roamed the Earth.

The first dinosaurs to be studied were large, which is why Sir Richard Owen gave them the name dinosaurs (Greek for "terrible lizards") in 1842. Many famous dinosaur discoveries came from the northern plains of the United States. In the late 19th century, almost 150 new dinosaur species were revealed by the excavations and reconstructions of American paleontologists Edward Drinker Cope and Othniel Charles Marsh. They began as collaborators but ended up in a three-decade intense rivalry. Marsh did not perform very much fieldwork. One account claims that he visited a spot where dinosaur bones were lying about like logs but did not recognize them. Marsh was, however, rich enough to buy fossils and to hire field workers. His uncle, George Peabody, endowed a museum at Yale specifically so that Marsh could study dinosaurs. Cope, who was also wealthy, spent much productive time in the field. He worked in Montana at the same time that General Custer's army was being annihilated nearby. When suspicious Natives checked out his camp, he entertained them by taking out his false teeth until they went away. A schoolteacher who found some dinosaur bones genially informed both Marsh and Cope about them. Cope sent the schoolteacher money and told him to ask Marsh to send all the specimens back to him. The rivalry grew intense enough that the excavators of the Marsh and Cope camps threw rocks at one another. Cope lost his money, but not his ego, in financial speculation, dying poor. He willed his body to a museum, hoping to have his skeleton used as the "type specimen" to represent the human species. When the skeleton was prepared, it was found to have syphilitic lesions and was unsuitable as a type specimen.

Most of what scientists know about dinosaurs comes from the study of fossilized bones, in the tradition of Cope and Marsh. Recently this has included the microscopic study of the spaces left by blood vessels in the bones. . .

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