The White tailed deer(Odocoileus virginianus), also known as the Virginia deer, or simply as the whitetail, is a medium-sized deer found throughout most of the continental United States, southern Canada, Mexico, Central America and northern portions of South America as far south as Peru.

The species is most common east of the American cordillera, and is absent from much of the western United States, including Nevada, Utah, and California (though its close relatives, the mule deer and black-tailed deer, can be found there). It does, however, survive in aspen parklands and deciduous river bottomlands within the Central and Northern Great Plains, and in mixed deciduous riparian corridors, river valley bottomlands, and lower foothills of the Northern Rocky Mountain Regions from Wyoming to Southeastern British Columbia. The conversion of land adjacent to the Northern Rocky Mountains into agriculture use and partial clear-cutting of coniferous trees (resulting in widespread deciduous vegetation) has been favorable to the white-tailed deer.

The westernmost population, the Columbian white-tailed deer once was widespread in the mixed forests along the Willamette River (Willamette Valley Forests Ecoregion) and Cowlitz River Valleys of Western Oregon and Southwestern Washington (endangered).

There are also populations of Arizona (coues) and Carmen Mountains (carminis) white-tailed deer that inhabit the mountain mixed deciduous/pine forests of Arizona, New Mexico, and West Texas extending southwards into Mexico.

As a result of introductions, white-tailed deer are found also in localised areas of northern Europe such as Finland. Smaller populations are localized in the Czech Republic.

White-tailed deer are generalists and can adapt to a wide variety of habitats. Although most often thought of as forest animals depending on relatively small openings and edges, white-tailed deer can equally adapt themselves to life in more open savanna and even sage communities as in Texas and in the Venezuelan llanos region. These savanna adapted deer have relatively large antlers in proportion to their body size and large tails. Also, there is a noticeable difference in size between male and female deer of the savannas.

The deer's coat is a reddish-brown in the spring and summer and turns to a grey-brown throughout the fall and winter. The deer can be recognized by the characteristic white underside to its tail, which it shows as a signal of alarm by raising the tail during escape.

The male (also known as a buck) usually weighs from 130 to 220 pounds (60 to 100 kg) but, in rare cases, animals in excess of 350 pounds (160 kg) have been recorded. The female (doe) usually weighs from 90 to 130 pounds (40 to 60 kg), but some can weigh as much as 165 to 175 pounds (75 or 80 kg).

Males one year of age or older have antlers. Bucks with very small antlers, about 3 in (7 cm) or less, are often termed "button bucks". Some may even have their antler pedicles hidden in the hair and can be mistaken for a doe. Antlers begin to grow in late spring, covered with a highly vascularised tissue known as velvet. Bucks either have a typical or non-typical antler arrangement. Typical antlers are symmetrical on both sides and the points grow straight up off the main beam. Non-typical antlers are asymmetrical and the points may project at any angle from the main beam. These descriptions are not the only limitations for typical and atypical antler arrangement. The Boone and Crockett or Pope & Young scoring systems also define relative degrees of typicality and atypicality by procedures to measure what proportion of the antlers are asymmetrical. Therefore, bucks with only slight asymmetry will often be scored as "typical". A buck's inside spread can be anywhere from 3–25 in (8–64 cm). Bucks shed their antlers when all females have been bred, from late December to February.

Females enter estrus, colloquially called the rut, in the fall, normally in late October or early November, triggered mainly by declining photoperiod. Sexual maturation of females depends on population density. Females can mature in their first year, although this is unusual and would occur only at very low population levels. Most females mature at one or, sometimes, two years of age.

Males compete for the opportunity of breeding females. Sparring among males determines a dominance hierarchy. Bucks will attempt to copulate with as many females as possible, losing physical condition since they rarely eat or rest during the rut. The general geographical trend is for the rut to be shorter in duration at increased latitude.

Females give birth to one, two or even possibly three spotted young, known as fawns in mid to late spring, generally in May or June. Fawns lose their spots during the first summer and will weigh from 44 to 77 pounds (20 to 35 kg) by the first winter. Male fawns tend to be slightly larger and heavier than females.

Whitetails communicate in many different ways including sounds, scent, and marking. All whitetail deer are capable of producing audible noises, unique to each animal. Fawns release a high pitched squeal, known as a bleat, to call out to their mothers. Does also bleat, as well as grunt. Grunting produces a low, guttural sound that will attract the attention of any other deer in the area. Both does and bucks snort, a sound that often signals danger. As well as snorting, bucks also grunt at a pitch thats gets lower with maturity. Bucks are unique, however, in their grunt-snort-wheeze pattern that often shows aggression and hostility.

Whitetails possess many glands that allow them to produce scents, some of which are so potent they can be detected by the human nose. Three major glands are the orbital, tarsal, and metatarsal glands. Orbital glands are found on the head, and scent is deposited from them by rubbing the head, often the area around the eyes, on hanging twigs. The tarsal glands are found on the lower outside of each hind leg. Scent is deposited from these glands when deer walk through and rub against vegetation. The metatarsal glands, found on the inside "knee" of each hind leg, are the most potent.

During the breeding season, deer will rub-urinate, a process during which a deer squats while urinating so that urine will run down the insides of the deer's legs. The deer then rubs its metatarsal glands together, rubbing the urine into the tuft of hair found at this location. Secretions from the metatarsal gland mix with the urine and bacteria to produce a strong smelling odor. Also in breeding season, does release hormones and pheromones that tell bucks the doe is in heat and able to breed.

Markings are a very obvious way that whitetail communicate. Although bucks do most of the marking, does visit these locations often. One form of marking is known as rubbing. To make a rub, a buck will use its antlers to strip the bark off of small diameter trees, helping to mark his territory and polish his antlers. Also to help mark territory, bucks will make scrapes. Often occurring in patterns known as scrape lines, scrapes are areas where a buck has used its front hooves to expose bare earth. Bucks usually then rub-urinate into these scrapes and scrapes are often found under twigs that have been marked with scent from orbital glands.

Commercial exploitation, unregulated hunting and poor land-use practices, including deforestation severely depressed deer populations in much of their range. For example, by about 1930, the U.S. population was thought to number about 300,000. After an outcry by hunters and other conservation ecologists, commercial exploitation of deer became illegal and conservation programs along with regulated hunting were introduced. Recent estimates put the deer population in the United States at around 30 million. Conservation practices have proved so successful that, in parts of their range, the white-tailed deer populations currently far exceed their carrying capacity and the animal may be considered a nuisance. Motor vehicle collisions with deer are a serious problem in many parts of the animal's range, especially at night and during rutting season, causing injuries and fatalities among both deer and humans. At high population densities, farmers can suffer economic damage by deer depredation of cash crops, especially in maize and orchards.

The species is the state animal of Arkansas, Illinois, Mississippi, New Hampshire, Ohio, Pennsylvania, Michigan, South Carolina, and Wisconsin, as well as the provincial animal of Saskatchewan. The profile of a White-tailed deer buck caps the Vermont coat-of-arms and can be seen in the Flag of Vermont and in stained glass at the Vermont State House. Texas is home to the most white-tailed deer of any other U.S. state or Canadian province, with an estimated population of over four million. Notably high populations of white-tailed deer occur in the Edwards Plateau of Central Texas. Michigan, Minnesota, Mississippi, New Jersey, New York, and Pennsylvania also boast high deer densities. In many U.S. states and Canadian provinces, hunting for white-tailed deer is deeply ingrained in local cultures and is central to the economy of many rural areas.

Since the second half of the nineteenth century, white-tailed deer have been introduced to Europe. In 1884, one of the first hunts of white-tailed deer was conducted in Opocno and Dobríš (Brdy mountains area), in what is now the Czech Republic. A population of white-tailed deer in the Brdy area remains stable today. In 1935, white-tailed deer were introduced to Finland. The introduction was successful, and the deer have recently begun spreading through northern Scandinavia and southern Karelia, competing with, and sometimes displacing, native fauna. The current population of some 30,000 deer originate from four animals provided by Finnish Americans from Minnesota.

There is a population of white-tailed deer in the state of New York that is entirely white (except for areas like their noses and toes) - not albino - in color. The former Seneca Army Depot in Romulus, New York, has the largest known concentration of white deer. Strong conservation efforts have allowed white deer to thrive within the confines of the depot.

In western regions of the United States and Canada, the white-tailed deer range overlaps with those of the black-tailed deer and mule deer. In the extreme north of the range, their habitat is also used by moose in some areas. White-tails may occur in areas that are also exploited by elk (wapiti) such as in mixed deciduous river valley bottomlands and formerly in the mixed deciduous forest of Eastern United States. In places such as Glacier National Park in Montana and several national parks in the Columbian Mountains (Mount Revelstoke National Park) and Canadian Rocky Mountains (e.g., Yoho National Park and Kootenay National Park), white-tailed deer are shy and more reclusive than the coexisting mule deer, elk, and moose.

The whitetail deer is a ruminant, which means it has a four-chambered stomach. Each chamber has a different and specific function that allows it to quickly eat a variety of different food, digesting it at a later time in a safe area of cover.

Whitetail deer eat large varieties of food, commonly eating legumes and foraging on other plants, including shoots, leaves, cactus, and grasses. They also eat acorns, fruit, and field corn or any kind of corn. Their special stomach allows them to eat some things that humans cannot, such as mushrooms that are poisonous to humans. Their diet varies in the seasons according to availability of food sources. Whitetail deer will also eat meat, to a lesser degree.

Until recently, some taxonomists have attempted to separate white-tailed deer into a host of subspecies, based largely on morphological differences. Genetic studies, however, suggest that there are fewer subspecies within the animal's range as compared to the 30 to 40 subspecies that some scientists described in the last century. The Florida Key deer, O. virginianus clavium, and the Columbian white-tailed deer, O. virginianus leucurus, are both listed as endangered under the U.S. Endangered Species Act. The dominant subspecies across the deer's range is the Virginia white-tail, O. virginianus virginianus which is also the type species for the Odocoileus genus. The White-tailed deer species has tremendous genetic variation and is adaptable to several environments. Several local deer populations, especially in the Appalachian and Piedmont regions of the eastern United States, are descended from white-tailed deer transplanted from other areas. Some of these deer may have been from northern mixed forests in the Great Lakes region, or from more open savannas and riparian bottomlands in the midwest and Texas, yet are also quite at home in the Appalachians and Piedmont. These deer over time have intermixed with the local indigenous deer populations.

Central and South America have a complex number of white-tailed deer subspecies that range from southern Mexico as far south as Peru. This list of subspecies of deer is more exhaustive than its North American counterpart and is also questionable, but populations are difficult to study due to over-hunting in many parts and lack of protection. Some areas no longer carry deer, so it is difficult to assess the genetic difference of these animals. Central American white-tailed deer prefer tropical dry deciduous forests, seasonal mixed deciduous forests, and savanna habitats over dense rainforests and cloud forests.

South American subspecies of white-tailed deer live in two types of environments. The first is found in the savannas, dry deciduous forests, and riparian corridors of southern Venezuela and eastern Colombia. The other is the higher elevation mountain grassland/mixed forest ecozones in the Andes Mountains, from Venezuela to Bolivia and Peru. The Andean white-tailed deer seem to retain gray coats due to the colder weather at high altitudes, whereas the lowland savanna forms retain the reddish brown coats. South American white-tailed deer, like those in Central America, generally avoid dense rainforests and cloud forests.