[1] Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids.

Turbines and propellers (and sometimes fans and pumps) use a number of rotating fins, also called foils, wings, arms or blades.

In 2011, researchers using volumetric imaging techniques were able to generate "the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes".

During courtship, the female cichlid, Pelvicachromis taeniatus, displays a large and visually arresting purple pelvic fin.

For this manoeuvrability is more important than straight line speed, so coral reef fish have developed bodies which optimize their ability to dart and change direction.

[38] The pectoral and pelvic fins of many reef fish, such as butterflyfish, damselfish and angelfish, have evolved so they can act as brakes and allow complex maneuvers.

[39] Many reef fish, such as butterflyfish, damselfish and angelfish, have evolved bodies which are deep and laterally compressed like a pancake, and will fit into fissures in rocks.

[39] Aristotle recognised the distinction between analogous and homologous structures, and made the following prophetic comparison: "Birds in a way resemble fishes.

There is an old theory, proposed by anatomist Carl Gegenbaur, which has been often disregarded in science textbooks, "that fins and (later) limbs evolved from the gills of an extinct vertebrate".

[51][52] In 2011, researchers at Monash University in Australia used primitive but still living lungfish "to trace the evolution of pelvic fin muscles to find out how the load-bearing hind limbs of the tetrapods evolved.

"[53][54] Further research at the University of Chicago found bottom-walking lungfishes had already evolved characteristics of the walking gaits of terrestrial tetrapods.

[55][56] In a classic example of convergent evolution, the pectoral limbs of pterosaurs, birds and bats further evolved along independent paths into flying wings.

Even with flying wings there are many similarities with walking legs, and core aspects of the genetic blueprint of the pectoral fin have been retained.

Recent DNA analysis suggests that cetaceans evolved from within the even-toed ungulates, and that they share a common ancestor with the hippopotamus.

The forelimbs became flippers and, in pinnipeds, the hind limbs became a tail terminating in two fins (the cetacean fluke, conversely, is an entirely new organ).

"This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design.

These structures are all the more remarkable because they evolved from nothing — the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor.

[68] In 2005, the Sea Life London Aquarium displayed three robotic fish created by the computer science department at the University of Essex.

"Robotic devices also facilitate three-dimensional kinematic studies and correlated hydrodynamic analyses, as the location of the locomotor surface can be known accurately.

And, individual components of a natural motion (such as outstroke vs. instroke of a flapping appendage) can be programmed separately, which is certainly difficult to achieve when working with a live animal.