Armored dinosaurs were some of the earliest dinosaurs named, including the ankylosaur Hylaeosaurus in 1833 and Stegosaurus in 1877. But these armored dinosaurs, or thyreophorans, have been the least-studied dinosaurs because they lack the visceral appeal of Tyrannosaurus and the fossil abundance of ceratopsians and hadrosaurs. The incredible diversity of armored dinosaurs has only recently been appreciated, owing to the discovery of new stegosaurs in the Jurassic of China and the United States, and of new ankylosaurs from the Lower Cretaceous of North America. These discoveries have been the impetus in recent years for renewed interest in thyreophorans. The Armored Dinosaurs brings together the latest studies by an international group of dinosaur paleontologists and provides descriptions of the original specimens of Hylaeosaurus and Stegosaurus, names new thyreophorans, and redescribes historically important specimens from Europe.