A flying mammal of the order Chiroptera. There are two sub-orders of bats, the Megachiroptera or flying foxes and the Microchiroptera which are smaller insectivorous bats. The wings of the bat evolved from the forelimbs; the finger bones are greatly elongated and joined to the forearm by a membrane, while the thumb is shaped into a claw which is used for catching insects or hanging on to surfaces. Bats have large ears, which helps them to locate objects by sonar - they can catch two flies in a second or detect and avoid a wire of diameter 0.1 mm. The head of a bat is usually rodentlike or foxlike. Bats vary greatly in size: the largest species, Pteropus vampyrus, has a wing span of 1.5 m, but the smallest bat, Craeseonycteris thonglongyai, has a wing span of only 15 cm.

Bats are found worldwide and usually live in colonies. They hunt at night and roost during the day. Some species roost in isolated locations such as caves, while others like the heat and dryness of external roosts such as tree trunks. Many species hibernate during the winter, and have to migrate between summer roosts and winter places of hibernation (such as a cave where the temperature is cool but not below freezing). Bats often form separate colonies for the different sexes, or clusters of each sex within a mixed-sex colony. Pregnant females go to separate "nursery roosts", which are sometimes chosen because they are hotter than average. Small bats suckle their young for 5-6 weeks, larger flying foxes for 5 months.

The commonest species of bats eat flying insects. The flying foxes eat fruit, and some rarer species of bat eat flowers, fish or small animals, while vampire bats live on the blood of larger birds and mammals. Bats are long-lived (some have lived for over 20 years) perhaps because roosting in secure places protects them from predators and their sonar makes them good at hunting and detecting threats.