The size of the genus has been considerably reduced as a result of work during the 1990s and 2000s, with genera like Arielulus, Hypsugo, Falsistrellus, Neoromicia, Parastrellus, Perimyotis, Scotozous and Vespadelus being split off. Still, molecular evidence suggests that the genus is not monophyletic. Several other genera in the subfamily Vespertilioninae have also been merged with Pipistrellus in previous classifications. Species in the genus may be referred to as "pipistrelles" or "pipistrelle bats", though these terms are also used for other genera.
They are somewhat distinguished from their much larger cousins, the noctule bats Nyctalus by their weak, fluttery flight reminiscent more of a butterfly than anything else, though a few species are more direct in their flight.