Vespertilionidae is a family of microbats, of the order Chiroptera, flying, insect-eating mammals variously described as the common, vesper, or simple nosed bats. The vespertilionid family is the most diverse and widely distributed of bat families, specialised in many forms to occupy a range of habitats and ecological circumstances, and it is frequently observed or the subject of research. The facial features of the species are often simple, as they mainly rely on vocally emitted echolocation. The tails of the species are enclosed by the lower flight membranes between the legs. Over 300 species are distributed all over the world, on every continent except Antarctica. It owes its name to the genus Vespertilio, which takes its name from a word for bat, vespertilio, derived from the Latin term vesper meaning 'evening'; they are termed "evening bats" and were once referred to as "evening birds".

Bats are flying mammals of the order Chiroptera. With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out digits covered with a thin membrane or patagium. The smallest bat, and arguably the smallest extant mammal, is Kitti's hog-nosed bat, which is 29–34 millimetres in length, 150 mm (6 in) across the wings and 2–2.6 g in mass. The largest bats are the flying foxes, with the giant golden-crowned flying fox reaching a weight of 1.6 kg and having a wingspan of 1.7 m.