Abstract: A method of forming a stiffened panel from first and second metal sheets, at least the first sheet being capable of both superplastic deformation and diffusion bonding, and also provided with at least one control region of different thickness compared with other regions of the sheet, includes the steps of:attaching the sheets together at a series of attachment lines across their faces, the attachment lines and the control region or regions being in predetermined relationship with one another, placing the attached sheets in a mould and heating to within that temperature range within which superplastic deformation and diffusion bonding takes place,urging those areas of the first sheet between the attachment lines away from the second sheet by a common differential pressure at a rate within that range of strain rates at which superplastic deformation occurs to form a series of cavities between the two sheets such that peripheral parts of those areas urged away from the second sheet form side walls of neighboring