A method of attaching an aircraft skin to an aircraft airframe, the method comprising: providing at least part of an aircraft airframe; measuring a surface of the at least part of the aircraft airframe; creating a digital model of the at least part of the aircraft airframe using the measurements of the surface of the at least part of the aircraft airframe; providing an aircraft skin; measuring a surface of the aircraft skin; creating a digital model of the aircraft skin using the measurements of the surface of the aircraft skin; digitally assembling the digital model of the at least part of the aircraft airframe with the digital model of the aircraft skin; using the digitally assembled digital model of the at least part of the aircraft airframe and the digital model of the aircraft skin, creating a digital model of a shim, the digital model of the shim substantially filling a gap between the digitally assembled digital model of the at least part of the aircraft airframe and the digital model of the aircraft skin; performing an Additive Manufacturing, AM, process to form the shim directly onto the surface of the at least part of the aircraft airframe, the AM process using the digital model of the shim; measuring a surface of the at least part of the aircraft airframe with the shim formed thereon; creating a digital model of the at least part of the aircraft airframe with the shim formed thereon; and attaching the aircraft skin to the at least part of the aircraft airframe with the physical shim formed thereon, wherein the attaching of the aircraft skin to the at least part of the aircraft airframe with the physical shim formed thereon uses the digital model of the at least part of the aircraft airframe with the shim formed thereon. providing at least part of an aircraft airframe; measuring a surface of the at least part of the aircraft airframe; creating a digital model of the at least part of the aircraft airframe using the measurements of the surface of the at least part of the aircraft airframe; providing an aircraft skin; measuring a surface of the aircraft skin; creating a digital model of the aircraft skin using the measurements of the surface of the aircraft skin; digitally assembling the digital model of the at least part of the aircraft airframe with the digital model of the aircraft skin; using the digitally assembled digital model of the at least part of the aircraft airframe and the digital model of the aircraft skin, creating a digital model of a shim, the digital model of the shim substantially filling a gap between the digitally assembled digital model of the at least part of the aircraft airframe and the digital model of the aircraft skin; performing an Additive Manufacturing, AM, process to form the shim directly onto the surface of the at least part of the aircraft airframe, the AM process using the digital model of the shim; measuring a surface of the at least part of the aircraft airframe with the shim formed thereon; creating a digital model of the at least part of the aircraft airframe with the shim formed thereon; and attaching the aircraft skin to the at least part of the aircraft airframe with the physical shim formed thereon, wherein the attaching of the aircraft skin to the at least part of the aircraft airframe with the physical shim formed thereon uses the digital model of the at least part of the aircraft airframe with the shim formed thereon.