An aircraft wing box comprises a pair of half ribs (18, 20) joined to form a rib extending from an upper wing skin (12) to a lower wing skin (10). Each half rib (18) comprises a base (19) and a plurality of projections (4) that extend away from the base (19), the projections (4) defining a series of troughs (2a) and peaks (4a). Respective projections of one half rib (18) overlap with respective projections of the other half rib (20) thereby defining overlapping portions (3), the half ribs (18, 20) being joined in the regions of the overlapping portions (3). There may be projections (4) shaped such that the overlapping portion (3b) defined by the two projections is separated from the neutral axis N of the rib (18, 20). The half ribs may be joined in the regions of the overlapping portions (3) by means of a fixing arrangement (7) that enables the position of joining of the half ribs to be adapted during initial assembly of the half-ribs (18, 20) to accommodate relative movement of the half ribs in a direction substantially parallel to the plane of one of the half ribs. The half ribs (18, 20) may be formed by deforming extruded metal sections. During assembly, in-wing systems (32) may be inserted before joining two wing-box halves.