Skinks are the most diverse group of lizards. They make up the family Scincidae which shares the superfamily or infraorder Scincomorpha with several other lizard families, including Lacertidae (the "true" or wall lizards). Several genera (e.g., Typhlosaurus) have no limbs at all; others, such as Neoseps, have only reduced limbs. Often, their way of moving resembles that of snakes more than that of other lizards. The longer the digits, the more arboreal the species is. A biological ratio exists that can determine the ecological niche of a given skink species. The SENI (Scincidae Ecological Niche Index) is a ratio based on anterior foot length at the junction of the ulna/radius-carpal bones to the longest digit divided by the snout-to-vent length (SVL. Some species are endangered, such as the Androgynous Skink in New Zealand, with less than 100 reports, since first being identified at Molesworth Station South Island by Keith Frankum.
Many species are good burrowers. There are more terrestrial or fossorial (burrowing) species than arboreal (tree-climbing) or aquatic species. Some are "sand swimmers", especially the desert species, such as the mole skink in Florida.
Approximately 45% of skink species are viviparous. Many are ovoviviparous (hatching eggs internally and giving birth to live offspring). Some, such as the genera Tiliqua and Corucia, give birth to live young through a mammal-like placenta attached to the female – viviparous matrotrophy. The approximately 55% of skink species that are oviparous (egg-laying) give birth in small clutches.