In aircraft construction, a longeron or stringeror stiffener is a thin strip of material, to which the skin of the aircraft is fastened. In the fuselage, longerons are attached to formers (also called frames) and run the longitudinal direction of the aircraft. In the wing or horizontal stabilizer, longerons run spanwise and attach to ribs.

Sometimes the terms "longeron" and "stringer" are used interchangeably. Historically, though, there is a subtle difference between the two terms. If the longitudinal members in a fuselage are few in number (usually 4 to 8), then they are called "longerons". The longeron system also requires that the fuselage frames be closely spaced (about every 4 to 6 in/10 to 15 cm). If the longitudinal members are numerous (usually 50 to 100), then they are called "stringers". In the stringer system the longitudinal members are smaller and the frames are spaced farther apart (about 15 to 20 in/38 to 51 cm). Generally, longerons are of larger cross-section when compared to stringers. On large modern aircraft the stringer system is more common because it is more weight-efficient, despite being more complex to construct and analyze. Some aircraft use a combination of both stringers and longerons.

Longerons often carry larger loads than stringers and also help to transfer skin loads to internal structure. Longerons nearly always attach to frames or ribs. Stringers often are not attached to anything but the skin, where they carry a portion of the fuselage bending moment through axial loading. It is not uncommon to have a mixture of longerons and stringers in the same major structural component.