Folding wings are design features incorporated into aircraft to reduce the amount of space a plane takes up on a deck or other surface, so that more aircraft can be parked in a given area. They are most frequently found on aircraft intended to operate from aircraft carriers, where stowage space on flight decks and hangar decks is at a premium; however, during World War I, in particular, the feature was also used on very large bomber aircraft operating from land, to allow them to fit into hangars designed for smaller aircraft. More recently wing folding has been incorporated into light civil aircraft, to allow their storage in car garages.

A folding wing has some disadvantages compared to a non-folding wing. It is heavier and has more complex connections for electrical, fuel, aerodynamic, and structural systems.

Historically there have always been aircraft small enough that a reduction in planform for shipboard storage was not considered necessary. These included most fighter aircraft of the 1910-1940 period, plus a handful of later types, including the Douglas SBD Dauntless and A4D/A-4 Skyhawk, and the Brewster F2A Buffalo (all USN types); the Mitsubishi A5M and Yokosuka D4Y (Japanese); and the Sea Harrier (British). All six are relatively compact designs. Carrier planes with non-folding wings which were still in use as of 2021 include the Dassault Rafale, the Lockheed Martin F-35B, and the McDonnell Douglas AV-8B Harrier. Larger aircraft, however, required a reduction in the amount of horizontal space they took up on a deck before they could be operated efficiently aboard a carrier.

Folding wings began to be incorporated into aircraft within a decade of the Wright Brothers' first flight in 1903. Short Brothers, the world's first series manufacturer of aircraft, developed and patented folding wing mechanisms for biplane ship-borne aircraft. The company developed a series of aircraft, known collectively as Short Folders, with the first patent for their folding wings being granted in 1913. Altogether ten models were characterized as Folders, of which one, the Short 184, was built in very substantial numbers (936), and others in some quantity. The folding mechanism on these aircraft involved the use of hinges on the trailing edge of each wing, at a break point not far from the fuselage; latches on the leading edge could be released, and the wings then swung backward, pivoting on the hinges, with their surfaces remaining horizontal, to end up adjacent to the fuselage, with their long axes parallel to the fuselage, giving the plane a very narrow planform. This became the standard method of wing folding for the next 20 years, and was even adopted for some early shipboard monoplanes (notably the Fairey Fulmar fighter of 1940).

This was not, however, the earliest folding-wing system to be incorporated into an airplane. In 1910 the Breguet company introduced its Type III aircraft, featuring a folding wing mechanism that was intended to reduce wingspan so that the plane could be transported by road (towed behind a car) or rail (on a flatcar) without disassembly. The Breguet mechanism was in many ways similar to the Grumman Sto-Wing mechanism of the 1940s, although it was, in some ways, more complex because the Breguet machine was a biplane. On the Type III the upper and lower wings on each side incorporated breaks very close to the fuselage. Main spars--the principal structural elements in the wings--located not far behind the leading edges of the wings incorporated hinges at the break points, with locking mechanisms near the trailing edges holding the wings in place while in flight. To fold the wings the outer panels of the wings could be unlocked and then rotated relative to the center section, with the spars serving as the axles around which the rotation took place. The folding process involved rotating the lower wing's leading edge downward 90 degrees, while the upper wing's leading edge was rotated upward 90 degrees, leaving the portions of the wings that lay behind the spars when in flight lying flat against one another, oriented vertically, much like overlapping shakes on the side of a house. The upper and lower wings could then be folded back together, swinging backward at the hinges in the spars to lie flush against the sides of the fuselage. The folding system did not even require interplane struts to be detached; these, too, were hinged, and lay flat against the outward-facing surfaces of the wing panels when folded, serving to hold the wings together even in the folded position.

As noted earlier, during World War I folding wings were incorporated into both carrier planes and large bombers. Among the machines in the latter category Britain’s Handley Page Type O series and the United States’ Martin MB series both incorporated Short-type folding wings. As for carrier planes, while fighter aircraft, as often as not, made do without folding wings, most larger aircraft incorporated the feature straight through the interwar period. British types fitted with Short-type folding wings included the Blackburn Ripon, Baffin, and Shark, and Fairey Swordfish and Albacore torpedo bombers; the Hawker Osprey fighter-reconnaissance plane; the Fairey IIIF spotter aircraft; the Supermarine Walrus shipboard flying boat; and the diminutive Parnall Peto fighter, intended for deployment aboard submarines. The Fairey Barracuda torpedo plane and the Fairey Fulmar fighter, both World War II monoplanes, also used this mechanism. Other military machines fitted with this sort of mechanism included the American Curtiss CS, Martin T3M and T4M, and the Japanese Mitsubishi B2M and Yokosuka B3Y and B4Y torpedo planes; the Curtiss SOC Seagull float-reconnaissance plane; numerous aircraft from the French Levasseur company; and the German land-based observation monoplane, the Fieseler Fi 156 Storch. A number of planes aimed at the civilian market, including the Parnall Elf of 1929 and numerous 21st century light planes (including many gliders), have also incorporated this type of wing-folding, to make stowage in home facilities (car garages) possible.

In the 1930s, with monoplanes coming into service, alternate types of folding wing began to appear. One type that became common incorporated a more literal folding of the wings: wings were hinged at break points about a third of the way out from the fuselage toward the tip, so that the outer panels could fold upward, leaving the wingtips close together and elevated over the fuselage of the plane, with the wingspan drastically reduced, though the height was marginally increased. This mechanism appears to have first been used on the Douglas TBD Devastator torpedo bomber, first flown in 1935, and the Vought SB2U Vindicator dive-bomber, first flown in early 1936, and has been seen on other carrier planes worldwide for ever since.

On some jets (the McDonnell-Douglas F/A-18 Hornet and the Soviet Yakovlev Yak-38 being notable examples) the wings are short enough that, even though some reduction in plan through folding was deemed desirable by the designers, all that was necessary was to provide for short segments of the wing tips to be folded up to a vertical position. Another refinement, which first appeared on Grumman's S2F (S-2) Tracker of 1954, and was later used on its C-1 Trader derivative and the Lockheed S3 Viking multirole aircraft of the 1970s, involved the incorporation of asymmetric hinging on the wings: all these were high-wing aircraft, and the asymmetric hinging allowed the wing panels, when folded, to extend all the way across the fuselage, with both lying nearly flat across the center section, with one in front of the other. This allowed them to fold at break points nearer the fuselage than in earlier aircraft, so that the width as well as the height of the stowed aircraft were minimized.