any of more than 2,050 living species of mammals (mammal) characterized by upper and lower pairs of ever-growing rootless incisor teeth. Rodents are the largest group of mammals, constituting almost half the class Mammalia's approximately 4,660 species. They are indigenous to every land area except Antarctica, New Zealand, and a few Arctic and other oceanic islands, although some species have been introduced even to those places through their association with humans. This huge order of animals encompasses 27 separate families, including not only the “true” rats (rat) and mice (mouse) (family Muridae) but also such diverse groups as porcupines (porcupine), beavers (beaver), squirrels (squirrel), marmots (marmot), pocket gophers (pocket gopher), and chinchillas (chinchilla).

All rodents possess constantly growing rootless incisors that have a hard enamel layer on the front of each tooth and softer dentine behind. The differential wear from gnawing creates perpetually sharp chisel edges. Rodents' absence of other incisors and canine teeth results in a gap, or diastema, between incisors and cheekteeth, which number from 22 (5 on each side of the upper and lower jaws (jaw)) to 4, may be rooted or rootless and ever-growing, and may be low- or high-crowned. The nature of the jaw articulation ensures that incisors do not meet when food is chewed and that upper and lower cheekteeth (premolars and molars) do not make contact while the animal gnaws. Powerful and intricately divided masseter muscles, attached to jaw and skull in different arrangements, provide most of the power for chewing and gnawing.

The range in body size between the mouse (18 grams 【0.64 ounce】, body 12 cm 【4.7 inches】 long) and the marmot (3,000 grams, body 50 cm long) spans the majority of living rodents, but the extremes are remarkable. One of the smallest is Delany's swamp mouse (Delanymys brooksi), associated with bamboo in the marshes and mountain forests in Africa. It weighs 5 to 7 grams, and the body is 5 to 6 cm long. The largest is the capybara (Hydrochoerus hydrochaeris) of Central and South America, which weighs 35 to 66 kg and stands 50 to 60 cm at the shoulder, with a body 100 to 135 cm long. Some extinct species were even larger, attaining the size of a black bear or small rhinoceros. The largest rodent ever recorded, Josephoartigasia monesi, lived some two to four million years ago, during the Pliocene Epoch; by some estimates it grew to a length of about 3 metres and weighed nearly 1,000 kg.

Rodents have lived on the planet for at least 56 million years and modern humans for less than one million, but the consequences of their interactions during that short overlap of evolutionary time have been profound. For rodents, early humans were just another predator to avoid, but with Homo sapiens' (Homo sapiens) transition from nomadic hunting and gathering to sedentary agricultural practices, humans became a reliable source of shelter and food for those species having the innate genetic and behavioral abilities to adapt to man-made habitats. The impact of these species upon human populations ranges from inconvenient to deadly. Crops are damaged before harvest; stored food is contaminated by rodent waste; water-impounding structures leak from burrowing; and objects are damaged by gnawing. Certain species are reservoirs for diseases (human disease) such as plague, murine typhus, scrub typhus, tularemia, rat-bite fever, Rocky Mountain spotted fever, and Lassa fever, among others. Only a few species are serious pests (pest) or vectors of disease (see house mouse and rat), but it is these rodents that are most closely associated with people.

Various other rodents are beneficial, providing a source of food through hunting and husbandry (see cavy, cane rat, bamboo rat, paca, capybara, and woodchuck), apparel derived from their fur (see nutria and chinchilla), test animals for biomedical and genetic research (especially mice and rats), pleasure as household pets (see golden hamster, guinea pig, and gerbil), and insight on mammalian biology and evolutionary history.

Rodents may be diurnal, nocturnal, or sometimes active part of the day and night. Although some species are herbivorous, diets of most include vegetable and animal matter. Others are opportunistic generalists, and some are specialized predators, not only of arthropods (see grasshopper mouse) but sometimes of vertebrates. Food is either eaten where gathered or carried to burrows and stored (see pocket gopher, pocket mouse, African pouched rat, and hamster). Species living in arid habitats and on oceanic islands are able to obtain their water requirements from their food. A wide variety of shelters are used or constructed; these range from tree holes, rock crevices, or simple burrows to hidden nests on the forest floor, leaf and stick structures in tree crowns, mounds of cut vegetation built in aquatic environments, or complex networks of tunnels and galleries. Rodents may be active all year or enter periods of dormancy or deep hibernation. Breeding time and frequency, length of gestation, and litter size vary widely, but two of the most prolific are both associated with humans. The brown rat (Rattus norvegicus) can give birth to litters of up to 22 offspring, and the house mouse (Mus musculus) can produce up to 14 litters annually. Population size may remain stable or fluctuate, and some species, most notably lemmings (lemming), migrate (migration) when populations become excessively large.

The body form of tree squirrels (squirrel) may be the model for the earliest, and presumably generalized, rodents (genus Paramys). With their ability to adhere to bark with their claws, squirrels adeptly scamper up tree trunks, run along branches, and leap to adjacent trees; but they are equally agile on the ground, and some are capable swimmers. Burrowers are also represented in the form of long-tailed ground squirrels.

The specialized body forms of other kinds of rodents tie them closer to particular locomotor patterns and ecologies. Some strictly arboreal species have a prehensile tail; others glide from tree to tree supported by fur-covered membranes between appendages (see flying squirrel and anomalure). Highly specialized fossorial (burrowing) rodents, including blind mole rats (blind mole rat), blesmols (blesmol), and ground squirrels (ground squirrel), are cylindrical and furry with protruding, strong incisors, small eyes and ears, and large forefeet bearing powerful digging claws. Semiaquatic rodents such as beavers (beaver), muskrats (muskrat), nutrias (nutria), and water rats (water rat) possess specialized traits allowing them to forage in aquatic habitats yet den in ground burrows. Terrestrial leaping species, such as kangaroo rats (kangaroo rat), jumping mice (jumping mouse), gerbils (gerbil), and jerboas (jerboa), have short forelimbs, long and powerful hind limbs and feet, and a long tail used for balance. Body forms of some rodents converge on those in nonrodent orders, resembling shrews (shrew), moles (mole), hares (hare), pikas (pika), pigs (pig), or small forest deer. There is also convergence between distantly related groups of rodents in particular body forms and associated natural histories.

Regardless of body form, all rodents share the same basic tools that, as mammologists Emmons and Feer noted, “can be used to cut, pry, slice, gouge, dig, stab, or delicately hold like a pair of tweezers; they can cut grass, open nuts, kill animal prey, dig tunnels and fell large trees.”

As documented by fossils, the evolutionary history of rodents extends back 56 million years to the Late Paleocene Epoch in North America. Those species, however, are considered to have originated in Eurasia, so the origin of the order Rodentia is certainly older. However, lack of fossil evidence prior to the Late Paleocene makes the understanding of evolutionary relationships between rodents above the familial level a continuing quest. Specialists agree with the definitions of most families, but they historically have disagreed, and still do, about the arrangement of families into larger groups—namely, suborders. Past classifications either have omitted suborders altogether and grouped the families into superfamilies or have grouped the families into 2, 7, 11, or 16 suborders. The outline below follows the latest formal classification, which employs five suborders, and is based upon a combination of the classical arrangements of the jaw and associated musculature, histologic structure of incisor enamel layers, comparative anatomy of the head and postcranial skeletons and different organ systems, embryonic development of extraembryonic fetal membranes, and analyses of DNA. Some specialists recognize just two suborders, Sciurognathi and Hystricognathi, which were proposed in 1899 and were based on conformation of the lower jaw. But any arrangement is simply a hypothesis of relationships between rodent families that is continually being tested by discovery of new fossils, reanalyses of data from conventional sources, and new analyses of data from many different, unrelated sources.