The surface of a moulding tool, for receiving a first aircraft component, for example a spar, is shaped to correspond to the surface of a second aircraft component, for example a wing skin. A gap is defined between the tool and the first component. Resin is drawn into the gap by a suction pump. The resin fills the gap and cures to forma shim. The first and second components, which are in the form of composite material structures, are then joined together without any significant voids being formed therebetween.