Bats range in body size from as small as the 3-cm (1-in) long Kitti's hog-nosed bat (also called the bumblebee bat), to as large as the 41-cm (16-in) long Malayan flying fox, which has a wingspan of 1.7-m (5.6-ft). The Kitti's hog-nosed bat is the world's smallest mammal, even smaller than a shrew. Body weights of bats range from 2-g (about 0.07-oz) to more than 1.3-kg (2.9-lb).

In most instances, the sexes of adult bats are easily distinguished. Adult males have a conspicuous penis, and adult females have an area around the nipples of their breasts where the hair has been worn away by nursing young. Many species also have subtle secondary characteristics-differences in size, vocalizations, and behaviors-that distinguish females from males.

Bat wings are supported internally by modified hand bones-a fact reflected in the name of the bat order, Chiroptera, which is Greek for "hand-wing." Each wing is made of a double layer of skin (called the wing membrane or patagium) that stretches between the bat's elongated finger bones and attaches to the side of the body and the hind leg. The clawed thumb is free of the wing, and the bat uses the thumb to cling to tree bark or the walls and ceiling of its roost.

Three pairs of flight muscles that attach to the bat's upper arms and chest produce the power for flight. When these strong muscles contract, they tug at the wing, producing a downstroke that lifts the bat skyward. The contractions of three other muscle pairs attached to the back cause the wing to raise in an upstroke, readying it for another downstroke. Flying bats have been clocked at speeds as fast as 100 km/h (60 mph) and observed at altitudes of 3 km (2 mi). Unlike most birds, bats are able to fly at relatively low speeds with extreme maneuverability, a feat achieved by the bat's extraordinary ability to vary the shape and curvature of their wings and thus dramatically vary their aerodynamic lift.

The tails on bats are extremely variable. Many bats have no tail or an extremely short tail that serves no purpose. Other bats, such as the mouse-tailed bats, have a tail that is nearly as long as the bat's body. In some species, such as the Natterer's bat, the tail is used as a net to catch insects, while other species use the tail as a pouch to hold insects until they can be eaten.

Bat fur is typically long, silky, and gray, but there are many exceptions-notably the pale white northern ghost bat of tropical America, the canary-hued yellow house bat of Africa, and the nearly black greater spear-nosed bat of Central America. The painted bat has scarlet fur and black and orange wings, which allows it to hide among flowers. There is even one species of hairless, or naked, bat. The primary function of bat fur is for warmth; however, some hairs (especially those on the face and other exposed areas) receive and transmit sensory impressions, much like the whiskers of cats or mice.

The faces of bats also vary considerably. Old World fruit bats have long, narrow, foxlike or doglike muzzles, while common vampire bats and their relatives possess short, piglike snouts. The ears of some species, including the aptly named long-eared bats, are as long as the bat's entire body. On the other hand, the black-bearded tomb bat has rather small ears. The shapes of bat ears are extremely varied. The wide ears of the Australian false vampire bat meet above the head and are fused. Many bats are able to turn their ears in the direction of faint noises. Bat hearing and its use in echolocation are highly developed.

Some bats have large, conspicuous eyes, while others may have small beady eyes. This variation suggests that vision plays different roles in the lives of various species. Despite the familiar expression, "blind as a bat," none of these animals are truly sightless. On the contrary, several bats can see better in dim light than do humans. However, only the Megachiroptera can perceive colors.

Like most mammals, bats have two sets of teeth. Depending upon the species, newborn bats are equipped with as many as 22 milk, or deciduous, teeth. These teeth are soon replaced with 20 to 28 permanent teeth. Bats have widely differing teeth patterns specialized for the diverse diets among the species, and teeth are often used to help identify bat species. Flesh-eating, or carnivorous, bats, such as the false vampire bats, have sharp and strong canine teeth for shearing flesh, and strong molars for crushing bones. Insect-eating bats have molars with sharp-edged surfaces that slide past each other in a scissorlike motion ideal for grinding the hard outer shells of insects. Fruit-eating bats have molars with large flat surfaces for grinding.