Varanidae was defined by Estes, de Queiroz and Gauthier (1988) as the clade containing the most recent common ancestor of Lanthanotus and Varanus and all of its descendants. A similar definition was formulated by Conrad et al. (2008), who defined Varanidae as the clade containing Varanus varius, Lanthanotus borneensis, and all descendants of their last common ancestor. Using one of these definitions leads to the inclusion of the earless monitor lizard (Lanthanotus borneensis) in the family Varanidae.
Lee (1997) created a different definition of Varanidae, defining it as the clade containing Varanus and all taxa more closely related to Varanus than to Lanthanotus; this definition explicitly excludes the earless monitor lizard from Varanidae. Whether Lanthanotus borneensis is included in or excluded from Varanidae depends on the author: for example, Vidal et al. (2012) classify the earless monitor lizard as a member of a separate family Lanthanotidae, while Gauthier et al. (2012) classify it as a member of Varanidae.
Monitor lizards are reputed to be among the most intelligent lizards. Most species forage widely and have large home ranges, and many have high stamina. Although most species are carnivorous, three arboreal species in the Philippines (Varanus olivaceus, Varanus mabitang, and Varanus bitatawa) are primarily frugivores. Among species of living varanids, the limbs show positive allometry, being larger in larger-bodied species, although the feet become smaller as compared with the lengths of the other limb segments.
Varanids possess unidirectional pulmonary airflow, including airsacs akin to those of birds.
^Estes, Richard; Kevin de Queiroz; Jacques Gauthier (1988). "Phylogenetic Relationships within Squamata". In Richard J. Estes. Phylogenetic Relationships of the Lizard Families: Essays Commemorating Charles L. Camp. Stanford University Press. p. 166. ISBN9780804714358.
^ abConrad J. (2008). "Phylogeny and systematics of Squamata (Reptilia) based on morphology". Bulletin of the American Museum of Natural History. 310: 1–182. doi:10.1206/310.1.
^Jacques A. Gauthier, Maureen Kearney, Jessica Anderson Maisano, Olivier Rieppel, Adam D.B. Behlke; Kearney; Maisano; Rieppel; Behlke (2012). "Assembling the Squamate Tree of Life: Perspectives from the Phenotype and the Fossil Record". Bulletin of the Peabody Museum of Natural History. 53 (1): 3–308. doi:10.3374/014.053.0101.CS1 maint: Multiple names: authors list (link)
^Massimo Delfino, Jean-Claude Rage, Arnau Bolet and David M. Alba (2013). "Synonymization of the Miocene varanid lizard Iberovaranus Hoffstetter, 1969 with Varanus Merrem, 1820". Acta Palaeontologica Polonica. in press. doi:10.4202/app.2012.0025.CS1 maint: Multiple names: authors list (link)
^Alexandra Houssaye, Nathalie Bardet, Jean–Claude Rage, Xabier Pereda Suberbiola, Baâdi Bouya, Mbarek Amaghzaz and Mohamed Amalik (2011). "A review of Pachyvaranus crassispondylus Arambourg, 1952, a pachyostotic marine squamate from the latest Cretaceous phosphates of Morocco and Syria". Geological Magazine. 148 (2): 237–249. doi:10.1017/S0016756810000580.CS1 maint: Multiple names: authors list (link)