Later, the number of specimens was increased by Reginald Walter Hooley.
In 1905, Baron Franz Nopcsa dedicated a study to Hypsilophodon, and in 1936 William Elgin Swinton did the same, on the occasion of the mounting of two restored skeletons in the British Museum of Natural History.
Most known Hypsilophodon specimens were discovered between 1849 and 1921 and are in the possession of the Natural History Museum that acquired the collections of Mantell, Fox, Hulke and Hooley.
These represent about twenty individual animals.
Apart from the holotype and paratype, the most significant specimens are: NHM R5829, the skeleton of a large animal; NHM R5830 and NHM R196/196a, both skeletons of juvenile animals; and NHM R2477, a block with a skull together with two separate vertebral columns.
Although this was the largest find, new ones continue to be made.

Modern research of Hypsilophodon began with the studies of Peter Malcolm Galton, starting with his thesis of 1967.
He and James Jensen briefly described a left femur, AMNH 2585, in 1975, and in 1979 formally coined a second species, Hypsilophodon wielandi, for the specimen.
The femur was diagnosed with two supposed minor differences from that of H. foxii.
The specimen was found in 1900 in the Black Hills of South Dakota, United States, by George Reber Wieland, who the species was named after.
Geologically, it comes from the Lakota Sandstone.
This species was seen at the time as indicative of a probable late land bridge between North America and Europe, and of the dinosaur fauna of both continents being similar.
Spanish Palaeontologist José Ignacio Ruiz-Omeñaca proposed that H. wielandi was not a species of Hypsilophodon but instead related to or synonymous with "Camptosaurus" valdensis from England, both species being dryosaurids.
Galton refuted this in his contribution to a 2012 book, noting the femurs of the two species to be quite different, and that of H. wielandi to be unlike those of dryosaurs.
He, as well as others studies before and after Ruiz-Omeñaca's proposal, considered H. wielandi a dubious basal ornithopod, with H. foxii the only species in the genus.
Galton elaborated on the invalidity of the species in 2009, noting that the two supposed diagnostic characters were variable in both H. foxii and Orodromeus makelai, making the species dubious.
He speculated that it may belong to Zephyrosaurus, from a similar time and place, as no femur was known from that taxon.

Fossils from other locations, especially from the mainland of southern Great Britain, Portugal and Spain, have once been referred to Hypsilophodon.
However, in 2009 Galton concluded that the specimens from Great Britain proper were either indeterminable or belonged to Valdosaurus, and that the fossils from the rest of Europe were those of related but different species.
This leaves the finds on Isle of Wight, off the south coast of England, as the only known authentic Hypsilophodon fossils.
The fossils have been found in the Hypsilophodon Bed, a one metre thick marl layer surfacing in a 1200 metre long strip along the Cowleaze Chine parallel to the southwest coast of Wight, part of the upper Wessex Formation and dating to the late Barremian, about 126 million years old.
Reports that Hypsilophodon would be present in the later Vectis Formation, Galton in 2009 considered as unsubstantiated.
Later, the number of specimens was increased by Reginald Walter Hooley. In 1905, Baron Franz Nopcsa dedicated a study to Hypsilophodon, and in 1936 William Elgin Swinton did the same, on the occasion of the mounting of two restored skeletons in the British Museum of Natural History. Most known Hypsilophodon specimens were discovered between 1849 and 1921 and are in the possession of the Natural History Museum that acquired the collections of Mantell, Fox, Hulke and Hooley. These represent about twenty individual animals. Apart from the holotype and paratype, the most significant specimens are: NHM R5829, the skeleton of a large animal; NHM R5830 and NHM R196/196a, both skeletons of juvenile animals; and NHM R2477, a block with a skull together with two separate vertebral columns. Although this was the largest find, new ones continue to be made.
Modern research of Hypsilophodon began with the studies of Peter Malcolm Galton, starting with his thesis of 1967. He and James Jensen briefly described a left femur, AMNH 2585, in 1975, and in 1979 formally coined a second species, Hypsilophodon wielandi, for the specimen. The femur was diagnosed with two supposed minor differences from that of H. foxii. The specimen was found in 1900 in the Black Hills of South Dakota, United States, by George Reber Wieland, who the species was named after. Geologically, it comes from the Lakota Sandstone. This species was seen at the time as indicative of a probable late land bridge between North America and Europe, and of the dinosaur fauna of both continents being similar. Spanish Palaeontologist José Ignacio Ruiz-Omeñaca proposed that H. wielandi was not a species of Hypsilophodon but instead related to or synonymous with "Camptosaurus" valdensis from England, both species being dryosaurids. Galton refuted this in his contribution to a 2012 book, noting the femurs of the two species to be quite different, and that of H. wielandi to be unlike those of dryosaurs. He, as well as others studies before and after Ruiz-Omeñaca's proposal, considered H. wielandi a dubious basal ornithopod, with H. foxii the only species in the genus. Galton elaborated on the invalidity of the species in 2009, noting that the two supposed diagnostic characters were variable in both H. foxii and Orodromeus makelai, making the species dubious. He speculated that it may belong to Zephyrosaurus, from a similar time and place, as no femur was known from that taxon.
Fossils from other locations, especially from the mainland of southern Great Britain, Portugal and Spain, have once been referred to Hypsilophodon. However, in 2009 Galton concluded that the specimens from Great Britain proper were either indeterminable or belonged to Valdosaurus, and that the fossils from the rest of Europe were those of related but different species. This leaves the finds on Isle of Wight, off the south coast of England, as the only known authentic Hypsilophodon fossils. The fossils have been found in the Hypsilophodon Bed, a one metre thick marl layer surfacing in a 1200 metre long strip along the Cowleaze Chine parallel to the southwest coast of Wight, part of the upper Wessex Formation and dating to the late Barremian, about 126 million years old. Reports that Hypsilophodon would be present in the later Vectis Formation, Galton in 2009 considered as unsubstantiated.
Hypsilophodon