Abstract: A stiffened panel of the type formed by a method including subjecting two metal sheets, both capable of superplastic deformation and diffusion bonding, which are positioned face to face, to a bending and deforming process during which the sheets are joined to one another at a series of spaced substantially continuous joint lines across their faces, and during which parts of said sheets between the joint lines are superplastically deformed in a mould to form a series of cavities between the two sheets, portions of said sheets on respective sides of a joint line being moved to form sidewalls of two neighboring cavities, these sidewalls being urged to lie adjacent one another over substantial parts of their areas, so that they become diffusion bonded one to another to form a common sidewall of neighboring cavities, and having formed spaced-apart regions where the sheets have contacted opposite mould faces is further formed with an outer lip region in which an edged region of one sheet remains generally aligned w