A biplane is a fixed-wing aircraft with two main wings of similar spans, normally one mounted above, and the other level with, the underside of the fuselage. The upper wing normally overlaps the lower wing, and vertical or slightly raked slender struts are often positioned symmetrically on either side of the fuselage (connecting the rigid sections of the upper and lower wings into a stong box structure). When the upper and lower wing overlap only partially, this is known as stagger; it is designed to minimise aerodynamic interference between the two wings. Forward stagger (where the upper wing is further forward) is most common, but backward stagger has also been used, notably in Beechcraft Staggerwing.

Gloster Gladiator biplane.

Aircraft built with two main wings (or three in a triplane) can usually lift more than can a similarly sized monoplane of similar wing-span, but most biplanes also have a third horizontal surface, either a tailplane or a foreplane , to control the pitch, or angle of attack of the aircraft. Either or both of the main wings can support flaps or ailerons to assist lateral and speed control.

Biplanes were most successfully marketed in the early days of aviation when the wing sections used were very thin and consequently the wing structure needed to be strengthened by external bracing wires . The biplane configuration allowed the two wings to be braced against one another, increasing the structural strength. Another advantage was the more compact layout with a shorter wing span, which led to greater maneuverability. The big disadvantage of the biplane layout was that the two wings interfered with one another aerodynamically, each reducing the lift produced by the other. This meant that for a given wing area the biplane produced more drag and less lift than a monoplane. Once thicker wing sections and improved structural materials were introduced, removing the need for external bracing, monoplanes quickly superseded biplanes and the latter now exist only in specialist niche roles.

A variation on the biplane was the sesquiplane, where the (usually) lower wing was significantly smaller than the other, either in span, chord, or both. On occasion, the lower wing was only large enough to support the bracing struts for the upper wing. The name means "one-and-a-half wings".

Famous biplanes include the Avro Tutor , Beechcraft Staggerwing, Boeing Stearman, Bristol Bulldog, de Havilland Tiger Moth, Fairey Swordfish, Hawker Hart and Pitts Special. The Stearman is particularly associated with stunt flying with wing-walkers. Famous sesquiplanes include the Nieuport 17 and Albatros D.V .

Biplane is also the title of an autobiographical book by Richard Bach that chronicles his journey across the United States in an antique biplane, landing in farmers' fields and offering rides for $3 USD.