Thresher sharks are large mackerel sharks of the family Alopiidae found in all temperate and tropical oceans of the world; the family contains three extant species, all within the genus Alopias.

All three thresher shark species have been listed as vulnerable by the World Conservation Union since 2007 (IUCN). All three are popular big-game sport fish, and additionally they are hunted commercially for their meat, livers (for shark liver oil), skin (for shagreen) and fins (for use in delicacies such as shark-fin soup).

Despite being active predatory fish, thresher sharks do not appear to be a threat to humans.

The genus and family name derive from the Greek word ἀλώπηξ, alṓpēx, meaning fox. As a result, the long-tailed or common thresher shark, Alopias vulpinus, is also known as the fox shark. The common name is derived from a distinctive, scythe-like tail or caudal fin which can be as long as the body of the shark itself.

The three extant thresher shark species are all in the genus Alopias. The possible existence of a hitherto unrecognized fourth species was revealed during the course of a 1995 allozyme analysis by Blaise Eitner. This species is apparently found in the eastern Pacific off Baja California, and has previously been misidentified as the bigeye thresher. So far, it is only known from muscle samples from one specimen, and no aspect of its morphology has been documented.

Based on cytochrome b genes, Martin and Naylor (1997) concluded the thresher sharks form a monophyletic sister group to the clade containing the families Cetorhinidae (basking shark) and Lamnidae (mackerel sharks). The megamouth shark (Megachasma pelagios) was placed as the next-closest relative to these taxa, though the phylogenetic position of that species has yet to be resolved with confidence. Cladistic analyses by Compagno (1991) based on morphological characters, and Shimada (2005) based on dentition, have both corroborated this interpretation.

Within the family, an analysis of allozyme variation by Eitner (1995) found the common thresher is the most basal member, with a sister relationship to a group containing the unrecognized fourth Alopias species and a clade comprising the bigeye and pelagic threshers. However, the position of the undescribed fourth species was only based on a single synapomorphy (derived group-defining character) in one specimen, so some uncertainty in its placement remains.

Although occasionally sighted in shallow, inshore waters, thresher sharks are primarily pelagic; they prefer the open ocean, characteristically preferring water 550 metres (1,800 ft) and less. Common threshers tend to be more prevalent in coastal waters over continental shelves. Common thresher sharks are found along the continental shelves of North America and Asia of the North Pacific, but are rare in the Central and Western Pacific. In the warmer waters of the Central and Western Pacific, bigeye and pelagic thresher sharks are more common. A thresher shark was seen on the live video feed from one of the ROVs monitoring BP's Macondo oil well blowout in the Gulf of Mexico. This is significantly deeper than the 500 m (1,600 ft) previously thought to be their limit. A bigeye has also been found in the western Mediterranean, and so distribution may be wider than previously believed, or environmental factors may be forcing sharks to search for new territories.