The crag martins are five species of small passerine birds in the genus Ptyonoprogne of the swallow family. They are the Eurasian crag martin (P. rupestris), the pale crag martin (P. obsoleta), the dusky crag martin (P. concolor), the large rock martin (P. fuligula) and the red-throated rock martin (P. rufigula). They are closely related to each other, and have formerly sometimes been considered to be one species. They are closely related to the Hirundo barn swallows and are placed in that genus by some authorities. These are small swallows with brown upperparts, paler underparts without a breast band, and a square tail with white patches. They can be distinguished from each other on size, the colour shade of the upperparts and underparts, and minor plumage details like throat colour. They resemble the sand martin, but are darker below, and lack a breast band.

These are species of craggy mountainous habitats, although all five will also frequent human habitation. The two African rock martins and the south Asian dusky crag martin are resident, but the Eurasian crag martin is a partial migrant; birds breeding in southern Europe are largely resident, but some northern breeders and most Asian birds are migratory, wintering in north Africa or India. They do not normally form large breeding colonies, but are more gregarious outside the breeding season. These martins build neat mud nests under cliff overhangs or in crevices in their mountain homes, and have readily adapted to the artificial cliffs provided by buildings and motorway bridges. Up to five eggs, white with dark blotches at the wider end, may be laid, and a second clutch is common. Ptyonoprogne martins feed mainly on insects caught in flight, and patrol cliffs near the breeding site with a slow hunting flight as they seek their prey. They may be hunted by falcons and infected with mites and fleas, but their large ranges and populations mean that none of the crag martins are considered to be threatened, and all are classed as Least Concern on the IUCN Red List.

The genus Ptyonoprogne was introduced in 1850 by the German naturalist Ludwig Reichenbach but in his publication he did not list any species for his new genus. In 1857 the French natural-history dealer Émile Parzudaki placed a single species, Hirundo rupestris Scopoli, 1769 (the Eurasian crag martin) in the genus so this is now the type species. The genus name is derived from the Greek ptuon (πτύον), "a fan", referring to the shape of the opened tail, and Procne (Πρόκνη), a mythological girl who was turned into a swallow. The genus Ptyonoprogne is sister to the genus Hirundo.

The genus now contains five species. These are members of the swallow family of birds, and are placed in the subfamily Hirundininae, which comprises all swallows and martins except the very distinctive river martins. DNA sequence 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". Ptyonoprogne species construct a mud nest and therefore belong to the last group; They resemble the Hirundo species in that they make open cup nests, whereas Delichon martins build closed cups, and the Cecropis and Petrochelidon swallows, have retort-like closed nests with an entrance tunnel. The genus Ptyonoprogne is closely related to the larger swallow genus Hirundo into which it is often subsumed, but a DNA analysis showed that a coherent enlarged genus Hirundo should contain all the mud-builder genera. Although the nests of the Ptyonoprogne crag martins resembles those of typical Hirundo species like the barn swallow, the DNA research showed that if the Delichon house martins are considered to be a separate genus, as is normally the case, Cecropis, Petrochelidon and Ptyonoprogne should also be split off.

The small, pale northern subspecies of crag martin found in the mountains of North Africa and the Arabian peninsula is now usually split as the pale crag martin, Ptyonoprogne obsoleta. The remaining birds are now identified as Eurasian crag martin.

The following cladogram is based on a molecular phylogenetic study of the Hirundinidae by Drew Schield and collaborators that was published in 2024.

These martins are 12–15 cm (4.7–5.9 in) long with drab brown or grey plumage and a short square tail that has small white patches near the tips of all but the central and outermost pairs of feathers. The eyes are brown, the small bill is mainly black, and the legs are brownish-pink. The sexes are similar, but juveniles show pale edges to the upperparts and flight feathers. The species differ in plumage shades and size, Eurasian crag martin being significantly larger than the others. The flight is slow, with rapid wing beats interspersed with flat-winged glides. The songs of these birds are simple twitterings, and contact calls include a high-pitched twee or chi, chi, and a tshir or trrt call like that of the house martin.

These drab martins can only be confused with each other, or with sand martins of the genus Riparia. Even the smaller Ptyonoprogne species are slightly larger and more robust than the sand martin and brown-throated sand martin, and have the white tail spots which are absent from the Riparia martins. Where the ranges of Ptyonoprogne species overlap, the Eurasian crag martin is darker, browner and 15% larger than the rock martin, and larger and paler, particularly on its underparts, than the dusky crag martin. The white tail spots of the Eurasian crag martin are significantly larger than those of both its relatives. In the east of its range, the rock martin always has lighter, more contrasted underparts than the dusky crag martin.