Delichon is a small genus of passerine birds that belongs to the swallow family and contains four species called house martins. These are chunky, bull-headed and short-tailed birds, blackish-blue above with a contrasting white rump, and with white or grey underparts. They have feathering on the toes and tarsi that is characteristic of this genus. The house martins are closely related to other swallows that build mud nests, particularly the Hirundo barn swallows. They breed only in Europe, Asia and the mountains of North Africa. Three species, the common, Siberian and Asian house martins, migrate south in winter, while the Nepal house martin is resident in the Himalayas year-round.

The house martins nest in colonies on cliffs or buildings, constructing feather- or grass-lined mud nests. The typical clutch is two or three white eggs; both parents build the nest, incubate the eggs and feed the chicks. These martins are aerial hunters of small insects such as flies and aphids. Despite their flying skills the Delichon martins are sometimes caught by fast-flying birds of prey. They may carry fleas or internal parasites. None of the species are considered threatened, although widespread reductions in common house martin numbers have been reported from central and northern Europe. This decline is due to factors including poor weather, poisoning by agricultural pesticides, lack of mud for nest building and competition with house sparrows for nest sites.

The four Delichon species are members of the swallow family of birds, and are classed as members of the Hirundininae subfamily which comprises all swallows and martins except the very distinctive river martins. DNA studies suggest that there are three major groupings within the Hirundininae, broadly correlating with the type of nest built. The groups are the "core martins" including burrowing species like the sand martin, the "nest-adopters", which are birds like the tree swallow that utilise natural cavities, and the "mud nest builders". The Delichon species construct a closed mud nest and therefore belong to the latter group; they appear to be intermediate between the Hirundo and Ptyonoprogne species that make open cup nests, and the Cecropis and Petrochelidon swallows, which have retort-like closed nests with an entrance tunnel. The genetic evidence suggests a close relationship between Hirundo and Delichon, which is further supported by the frequency of interbreeding between two widespread species, the barn swallow and the common house martin, despite being their being in different genera. The suggested taxonomic sequence of the mud-building swallows has been recommended by at least two European taxonomic committees.

The genus Delichon was created by American naturalist Thomas Horsfield and British entomologist Frederic Moore in 1854 to accommodate the Nepal house martin that was first described by Moore in the same year, and is therefore the type species for the genus. The two other house martins were moved to Delichon from the genus Chelidon in which they had been placed up to that time. In 2021, the Siberian house martin was split from the common house martin based on morphological and vocal differences. The name of the genus, "Delichon", is an anagram of the Ancient Greek term χελιδὡν/chelidôn, meaning swallow.

The genus contains four similar species:

The common and Asian house martins have sometimes been considered to be a single species, although both breed in the western Himalayas without hybridising. There is also limited DNA evidence that suggests a significant genetic distance between these two martins.

Delichon is an Old World genus with all four species breeding only in the Northern Hemisphere. The common house martin is a widespread migrant breeder across Europe, north Africa and all northern temperate Asia to Kamchatka. It winters in tropical Africa. The Siberian house martin breeds in northeast Russia and winters in southern Asia. The Asian house martin breeds further south than the Siberian house martin in the mountains of central and eastern Asia; its nominate subspecies winters in Southeast Asia, but the races breeding in the Himalayas and Taiwan may just move from the high mountains to lower altitudes. The Nepal house martin is resident in the mountains of southern Asia.

The preferred habitat of the common and Siberian house martins is open country with low vegetation, such as pasture, meadows and farmland, and preferably near water, although it is also found in mountains up to at least 2,200 metres (7,200 ft) altitude. As the name implies, they readily nest on man-made buildings, and will breed even in city centres if the air is clean enough. The other two species favour mountainous country (and sea cliffs in the case of Asian house martin); they use buildings as nest sites less frequently than their northern relative. The wintering grounds of the two migrant species include a range of open country and hilly habitats.