A method of assembling parts to form an aircraft component, in which a first surface of a first part (for example a wing skin) is to be positioned in the assembly adjacent to a second surface of a second part (for example a rib foot). The method comprises the steps of taking measurements of the first part, taking measurements of the second part with a laser, and calculating the size and shape of the space that will be generated when the first and second parts are assembled. A shim of varying thickness in dependence upon the results of the calculating step is manufactured, for example by stereolithography, and the first and second parts are assembled with the shim positioned between the first and second parts, the varying thickness of the shim compensating for a varying spacing of the first and second surfaces.