This inclusive status was supported by some phylogenetic analyses from the 1990s and mid 2000s,[3][4] although there have also been many finding that the family is an unnatural grouping which should only include the type genus, Hypsilophodon, with the other genera being within clades like Thescelosauridae and Elasmaria.

[12][17] The family Hypsilophodontidae is formally defined in the PhyloCode by Daniel Madzia and colleagues in 2021 as "the largest clade containing Hypsilophodon foxii, but not Iguanodon bernissartensis and Rhabdodon priscus".

[16] Hypsilophodontidae was named originally in 1882 by Louis Dollo, as a family to include Hypsilophodon and other small ornithopods with a single row of teeth, four pedal digits, and a rhomboid sternum.

Laosaurus and Dryosaurus were not considered hypsilophodonts because of their lack of distinguishable features, as Swinton concluded that they were probably in the family Laosauridae, intermediate between Hypsilophodontidae and Iguanodontidae, and were probably synonyms of each other as well.

[20] Charles M. Sternberg (1940) considered there to be multiple genera within the family, all sharing fully enamelled teeth, divided into two subfamilies, Hypsilophodontinae and Thescelosaurinae.

[22] Fabrosaurus -> Echinodon Heterodontosaurus Ankylosauria Stegosauria Hadrosauridae Iguanodon -> Tenontosaurus Ceratopsia Stenopelix -> Psittacosaurus Camptosaurus leedsi -> Dysalotosaurus -> Dryosaurus -> Wealden ornithopod Camptosaurus Yaverlandia -> Stegoceras -> Pachycephalosaurus Parksosaurus Laosaurus minimus Thescelosaurus In 1992 David Weishampel and Ronald Heinrich reviewed the systematics and phylogenetics of Hypsilophodontidae.

[2] Hypsilophodon Tenontosaurus Rhabdodontidae Dryosauridae Camptosaurus Styracosterna In one analysis in her 2022 review of iguanodontian phylogenetic relationships, Karen E. Poole recovered a large Hypsilophodontidae as the sister taxon of Iguanodontia, which consisted of several "traditional" hypsilophodontids, as well as Thescelosauridae.

The Bayesian topology of her phylogenetic analyses is shown in the cladogram below:[23] Othnielosaurus Jeholosaurus Haya Hypsilophodon Leaellynasaura Gasparinisaura Macrogryphosaurus Zephyrosaurus Orodromeus Parksosaurus Thescelosaurus spp.