Ceratopsians is a group of herbivorous, beaked dinosaurs which thrived in what are now North America and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. Early members such as Psittacosaurus were small and bipedal. Later members, including ceratopsids like Centrosaurus and Triceratops, became very large quadrupeds and developed elaborate facial horns and a neck frill. While the frill might have served to protect the vulnerable neck from predators, it may also have been used for display, thermoregulation, the attachment of large neck and chewing muscles or some combination of the above. Ceratopsians ranged in size from 1 meter (3 ft) and 23 kilograms (50 lb) to over 9 meters (30 ft) and 5,400 kg (12,000 lb).

Pachycephalosauria ("Thick headed lizards") is a clade of ornithischian dinosaurs. Well-known genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex. Most lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls. In some species the skull roof is domed and several inches thick; in others it is flat or wedge-shaped. The dome may be surrounded by nodes (for instance, Pachycephalosaurus), spikes (Stygimoloch), or both (Dracorex).

Achelousaurus ("Achelous's lizard") is a genus of centrosaurine ceratopsid dinosaur from the Late Cretaceous Period of what is now North America. It was a quadrupedal herbivore with a parrot-like beak, a rough boss (raised bony area) on the snout and two more behind the eyes, and two horns on the end of its long bony neck frill. With a total body length of 6 meters (20 feet), Achelousaurus was a medium-sized ceratopsian.

Albertaceratops ("Alberta horned face") was a genus of centrosaurine horned dinosaur from the middle Campanian-age Upper Cretaceous Oldman Formation of Alberta, Canada, and Judith River Formation of Montana, USA. It is known from a single complete skull found in August 2001 and skull and postcranial fragments. This genus is unusual in combining long brow horns with an otherwise centrosaurine skull, as centrosaurines normally possess short brow horns. Over its nose was a bony ridge, and on its frill were two large outwardly-projecting hooks. A phylogenetic analysis carried out by its describer, Michael J. Ryan, found it to be the most basal centrosaurine.

Anchiceratops ("Near horned face") is a genus of chasmosaurine ceratopsid dinosaur from the Late Cretaceous Period of western North America. Like other ceratopsids, it was a quadrupedal herbivore with three horns on its face, a parrot-like beak, and a long frill extending from the back of its head. The two horns above the eyes were longer than the single horn on its snout, as in other chasmosaurines. Anchiceratops approached 20 feet (6 m) in length.

Centrosaurus ("Pointed lizard") is an herbivorous ceratopsian dinosaur from the late Cretaceous of North America, approximately 75 million years ago. Centrosaurus massive body was borne by stocky limbs, although at 18-20 ft (6m) it was not a particularly large dinosaur. Like other centrosaurines, Centrosaurus had a single large horn over the nose. It may curve forwards or backwards in different species. A pair of small horns is also found over the eyes while the frill of Centrosaurus was moderately long, with fairly large fenestrae and small hornlets along the outer edge.

Chasmosaurus is a genus of ceratopsid dinosaur from the Upper Cretaceous Period of North America. Its name means "opening lizard", referring to the large openings (fenestrae) in its frill. With a length of 5–6 metres (16–20 ft) and a weight of 3.6 tonnes (4.0 short tons), Chasmosaurus was a ceratopsian of average size. Like all ceratopsians, it was purely herbivorous. It was initially to be called Protorosaurus, but this name had been previously published for another animal.

Einiosaurus is a medium-sized centrosaurine ("Short-frilled") ceratopsian from the Upper Cretaceous (Campanian) Two Medicine Formation of northwestern Montana. Einiosaurus is typically portrayed with a low, strongly forward-curving nasal horn that resembles a bottle opener, though this may only occur in some adults. Supraorbital (over-the-eye) horns are low and rounded if present at all, as opposed to ceratopsids with prominent supraorbital horns such as Triceratops. A pair of large spikes projects backwards from the relatively small frill.

Homalocephale ("Even head") is a genus of dinosaur belonging to the pachycephalosaurid family, which lived during the late Cretaceous period. The genus was described in 1974 by Osmólska and Maryañska, and consists of a single species, the 3 metres (10 ft) long herbivore. Sporting a flat, wedge-shaped skull roof, H. calathocercos was different from other pachycephalosaurs. Nonetheless, the surface of the skull was greatly thickened, and it had been proposed that males competed in head-butting contests, similar to those of modern iguanas. The species is also noted for having an unusually broad pelvis, which lead some paleontologists to suggest that the wide hips were for giving birth to live young. Others have suggested that the width served to protect vital organs from harm during flank-butting. Homalocephale also had rather long legs, indicating a fast-moving gait.

Pachycephalosaurus ("Thick headed lizard") is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period (Maastrichtian stage) of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few extremely thick skull roofs.

Pachyrhinosaurus ("Thick-nosed reptile") is a genus of ceratopsid dinosaur from the Late Cretaceous period of North America. Instead of horns, the skull bears massive, flattened bosses, the largest being over the nose. These were probably used in butting and shoving matches, as in musk oxen. A single pair of horns grew from the frill and extended upwards. It appears that that both the shape and size of the frill was highly individualized, reliant on gender and perhaps other factors. Pachyrhinosaurus is most closely related to Achelousaurus.

Pentaceratops is a genus of ceratopsid dinosaur from the late Cretaceous Period of what is now North America. Its name means "five-horned face", in reference to its two long epijugal bones, spikes which protrude out sidewards from under its eyes, in addition to the three more obvious horns. Pentaceratops lived around 75-73 million years ago and measured about 8 m (27 ft) long, and has been estimated to have weighed around 5,500 kg (13,000 lb).

Prenocephale was a small pachycephalosaurid dinosaur genus from the Late Cretaceous (from the Campanian through to the Maastrichtian) and was similar in many ways to its close relative, Homalocephale. Prenocephale probably weighed around 130 kilograms (290 lb) and was around 2.4 metres (8 ft) long. Unlike the flattened wedge-shaped skull of Homalocephale, the head of Prenocephale was rounded and sloping. The dome had a row of small, bony spikes and bumps. It lived in what is now Mongolia; but in high upland forests, not the dry deserts of Mongolia today.

Protoceratops ("First Horned Face") is a genus of sheep-sized (1.5 to 2 m long) herbivorous ceratopsian dinosaur, from the Upper Cretaceous Period (Campanian stage) of what is now Mongolia. It was a member of the Protoceratopsidae, a group of early horned dinosaurs. Unlike later ceratopsians, however, it lacked well-developed horns and retained some primitive traits not seen in later genera.

Psittacosaurus ("'Parrot lizard") is a genus of psittacosaurid ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia, about 130 to 100 million years ago. It is notable for being the most species-rich dinosaur genus. At least ten extinct species are recognized from fossils found in different regions of modern-day China, Mongolia and Russia, with a possible additional species from Thailand.

Stegoceras ("Horned roof") was a genus of plant-eating ornithischian pachycephalosaurid dinosaur that lived in what is now North America during the Late Cretaceous period. It had an estimated length of up to 2 metres (6.6 ft).

Stygimoloch ("Horned devil from the river of death") is a genus of pachycephalosaurid dinosaur from the end of the Cretaceous period, roughly 65 million years ago. It is currently known from the Hell Creek Formation and Lance Formation of the Western Interior (United States), where it lived alongside Tyrannosaurus and Triceratops. It is a relatively large pachycephalosaur, with the skull being about 46 centimeters long (18 in). Among North American pachycephalosaurs, only Pachycephalosaurus is larger. Unlike other pachycephalosaurs, the domed skull is relatively small, slightly flattened from side to side, and pear-shaped; even when isolated this unusual dome can easily be distinguished from the broader, larger domes of Pachycephalosaurus.

Styracosaurus ("Spiked lizard"') was a genus of herbivorous ceratopsian dinosaur from the Cretaceous Period (Campanian stage), about 76.5 to 75.0 million years ago. It had four to six long horns extending from its neck frill, a smaller horn on each of its cheeks, and a single horn protruding from its nose, which may have reached dimensions of around 60 centimeters (2 ft) long and 15 centimeters (6 in) wide. The function or functions of the horns and frills have been the subject of debate for many years. Styracosaurus was a large dinosaur, reaching lengths of 5.5 meters (18 ft) and weighing nearly 3 tons. It stood about 1.8 meters (6 ft) tall. Styracosaurus possessed four short legs and a bulky body. Its tail was rather short. It also had a beak and flat cheek teeth, indicating that its diet was herbivorous. Like other ceratopsians, this dinosaur may have been a herd animal, traveling in large groups, as suggested by bonebeds.

Torosaurus ("Perforated lizard") was a genus of ceratopsid dinosaur. It had one of the largest skulls of any land animal known, reaching 2.6 meters (8.5 ft) in length. From head to tail, Torosaurus probably measured about 7.6 meters (25 ft) long and weighed an estimated 4 to 6 tonnes (4.4 to 6.6 tons).

Triceratops is a genus of herbivorous ceratopsid dinosaur which lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago (mya) in what is now North America. It was one of the last dinosaur genera to appear before the great Cretaceous–Tertiary extinction event. Individual Triceratops are estimated to have reached about 7.9 to 9.0 m (26.0–29.5 ft) in length, 2.9 to 3.0 m (9.5–9.8 ft) in height, and 6.1–12.0 tonnes (13,000-26,000 lb) in weight. The most distinctive feature is their large skull, among the largest of all land animals. It could grow to be over 2 m (7 ft) in length, and could reach almost a third of the length of the entire animal. It bore a single horn on the snout, above the nostrils, and a pair of horns approximately 1 m (3 ft) long, with one above each eye. To the rear of the skull was a relatively short, bony frill. Most other frilled dinosaurs had large fenestrae in their frills, while the frills of Triceratops were noticeably solid.