Strepsiptera (/strɛpˈsɪptərə/), from Ancient Greek στρέψις (strépsis), meaning "turning around", and πτερόν (pterón), meaning "wing", are an order of insects with eleven extant families that include about 600 described species. They are endoparasites of other insects, such as bees, wasps, leafhoppers, silverfish, and cockroaches. Females of most species never emerge from the host after entering its body, finally dying inside it. The early-stage larvae do emerge because they must find an unoccupied living host, and the short-lived males must emerge to seek a receptive female in her host. They are believed to be most closely related to beetles, from which they diverged 300–350 million years ago, but do not appear in the fossil record until the mid-Cretaceous around 100 million years ago.

The order is not well known to non-specialists, and the nearest they have to a common name is stylops, in reference to the genus Stylops. The name of the order translates to "twisted wing", giving rise to other common names used for the order, twisted-wing insects and twisted-winged parasites.

Adult males are rarely observed, although specimens may be lured using cages containing virgin females. Nocturnal specimens can also be collected at light traps.

Males of the Strepsiptera have wings, legs, eyes, and antennae, though their mouthparts cannot be used for feeding. Many have mouthparts modified into sensory structures. The males bear a superficial resemblance to flies. The forewings are modified into small club-shaped structures called halteres, which sense gyroscopic information. A similar organ exists in flies, though in that group the hindwings are modified instead, and the two groups are thought to have independently evolved the structures. The hindwings are generally fan-shaped, and have strongly reduced venation. The antennae are flabellate, and are covered in specialised chemoreceptors, likely to detect females over long distances.

Adult male Strepsiptera have eyes unlike those of any other insect, resembling the eyes found in the trilobite group Phacopina. Instead of a compound eye consisting of hundreds to thousands of ommatidia, that each produce a pixel of the entire image, the strepsipteran eyes consist of only a few dozen "eyelets" that each produce a complete image. These eyelets are separated by cuticle and/or setae, giving the cluster eye as a whole a blackberry-like appearance.

The females of Stylopidia, which includes 97% of all described strepsipteran species and all modern strepsipteran families except Mengenillidae and Bahiaxenidae, are not known to leave their hosts and are neotenic in form, lacking wings, legs, and eyes, but have a well sclerotised cephalothorax (fused head and thorax). Adult female mengenillids are wingless but are free living and somewhat mobile with legs and small eyes. This is probably also true for the bahiaxenids, though this has not been observed.

Newly hatched primary (first instar) larvae are on average 230 micrometres (1⁄128 in) in length, smaller than many single-celled organisms. They are highly mobile with well developed stemmata, which are able to distinguish color. The underside of the body is covered in minute hair-like structures (microtrichia), which allow the larvae to stick to wet surfaces via capillary action. At the back of the body are well developed large bristle-like cerci, which are attached to muscles, which allow the larvae to jump. The tarsal segment of their legs have structures which allow them to cling to their hosts. Later larval instars which develop inside the host are completely immobile.

Virgin females release a pheromone which the males use to locate them. Mating in at least some species is polyandrous, where the female mates with more than one male.