The Lepidosauria (/ˌlɛpɪdoʊˈsɔːriə/, from Greek meaning scaled lizards) is a superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata also includes lizards and snakes. Squamata contains over 9,000 species, making it by far the most species-rich and diverse order of non-avian reptiles in the present day. Rhynchocephalia was a formerly widespread and diverse group of reptiles in the Mesozoic Era. However, it is represented by only one living species: the tuatara (Sphenodon punctatus), a superficially lizard-like reptile native to New Zealand.

Lepidosauria is a monophyletic group (i.e. a clade), containing all descendants of the last common ancestor of squamates and rhynchocephalians. Lepidosaurs can be distinguished from other reptiles via several traits, such as large keratinous scales which may overlap one another. Purely in the context of modern taxa, Lepidosauria can be considered the sister taxon to Archelosauria, which includes Testudines (turtles), Aves (birds) and Crocodilia (crocodilians). Lepidosauria is encompassed by Lepidosauromorpha, a broader group defined as all reptiles (living or extinct) closer to lepidosaurs than to archosaurs.

Lepidosauromorpha is thought to have split from their sister group, Archelosauria, during the Permian period. The earliest members of Lepidosauromorpha date to the Early Triassic. The oldest known definitive lepidosaur is the rhynchocephalian Agriodontosaurus from the Helsby Sandstone Formation of the United Kingdom, dating to the upper Anisian stage of the Middle Triassic, approximately 244 to 241.5 million years ago. The next earliest rhynchocephalian, Wirtembergia, is known from the Ladinian stage of the Middle Triassic. Sophineta is known from older rocks in the Early Triassic, but its exact placement within the broader clade Lepidosauromorpha is uncertain and it may not be a true lepidosaur. While the lepidosaur Megachirella may represent a stem-group squamate from the Middle Triassic, the earliest modern members of the group are known from the Middle Jurassic. Squamates underwent a great radiation in the Cretaceous, while rhynchocephalians declined during the same time period.

Extant reptiles are in the clade Diapsida, named for the two pairs of temporal fenestrae present on the skull behind the eye socket. Until recently, Diapsida was said to be composed of Lepidosauria and their sister taxa Archosauria. The Lepidosauria is then split into Squamata and Rhynchocephalia. More recent morphological studies and molecular studies^{[excessive citations]} also place turtles firmly within Diapsida, even though they lack temporal fenestrations.

The reptiles in the Lepidosauria can be distinguished from other reptiles by a variety of characteristics. Lepidosaurs are suggested to be distinguished from more primitive lepidosauromorphs by the development of a conch on the quadrate, allowing for the development of a tympanic membrane in the ear (a trait lost in the tuatara, but present in early rhynchocephalians), as well as the development of a subolfactory process on the frontal bones of the skull.

The group Squamata includes snakes, lizards, and amphisbaenians. Squamata can be characterized by the reduction or loss of limbs. Snakes and legless lizards have evolved the complete loss of their limbs. The upper jaw of Squamates is movable on the cranium, a configuration called kinesis. This is made possible by a loose connection between the quadrate and its neighboring bones. Without this, snakes would not be able consume prey that are much larger than themselves. Amphisbaenians are mostly legless like snakes, but are generally much smaller. Three species of amphisbaenians have kept reduced front limbs and these species are known for actively burrowing in the ground. The tuatara and some extinct rhynchocephalians have a more rigid skull with a complete lower temporal bar closing the lower temporal fenestra formed by the fusion of the jugal and quadrate/quadratojugal bones, similar to the condition found in primitive diapsids. However early rhynchocephalians and lepidosauromorphs had an open lower temporal fenestra, without a complete temporal bar, so this is thought to be a reversion rather than retention. The temporal bar is thought to stabilise the skull during biting.

Male squamates have evolved a pair of hemipenises instead of a single penis with erectile tissue that is found in crocodilians, birds, mammals, and turtles. The hemipenis can be found in the base of the tail. The tuatara does not have a hemipenis, but instead has shallow paired outpocketings of the posterior wall of the cloaca.

Second, most lepidosaurs have the ability to autotomize their tails. However, this trait has been lost on some recent species. In lizards and rhynchocephalians, fracture planes are present within the vertebrae of the tail that allow for its removal. Some lizards have multiple fracture planes, while others just have a single fracture plane. The regrowth of the tail is not always complete and is made of a solid rod of cartilage rather than individual vertebrae. In snakes, the tail separates between vertebrae and some do not experience regrowth.