The clade Chiroptera includes two extant clades, Megachiroptera (Old World Fruit Bats) and Microchiroptera (echolocating bats). In addition, Chiroptera includes at least four extinct clades that are most closely related to Microchiroptera. There are over nine hundred extant species of bats (Koopman, 1993). Bats vary greatly in size. The smallest bat, Craseonycteris thonglongyai (Microchiroptera), weighs less than 2 g and has a wingspan of 12-13 cm, while the largest bats, those of the genus Pteropus (Megachiroptera), weigh up to 1.5 kg and may have a wing span over 2m (Fenton, 1992).

Bats are unique among mammals as they are the only group to have evolved true powered flight. Some other mammals such as "flying" squirrels and "flying" lemurs can glide through the air for long distances, but they are not capable of sustained flight. In contrast, bats can propel themselves with their wings, gaining and loosing altitude and flying for long periods.

Bats are nocturnal and usually spend the daylight hours roosting in caves, rock crevices, trees, or manmade structures such as houses and/or bridges. Some bats are solitary, while others are found in colonies that may include over a million individuals.

Activity begins around dusk, when bats leave the day roost and start feeding. The clade Chiroptera includes species with very diverse food preferences, including bats that eat either meat, insects, fish, fruit, nectar, or a variety of food types. Only three species of bats actually feed on blood Desmodontinae). Many bats remain at their feeding sites until just before dawn when they return to the day roost.

Classification outlines the higher-level classifications within Chiroptera.