Spline couplings are able to accommodate some misalignment, although this may result in fretting wear of the teeth. This can reduce coupling life or cause complete tooth fracture. Computational and experimental methods have been used to investigate the problem for straight, involute splines. Boundary element analyses of complete couplings have identified four contact regimes that may occur; frill tooth contact may exist, teeth may contact at ends alone and teeth may frilly separate during rotation. At large misalignments the coupling may topple leading to contact on both sides of the teeth. The regime is dependent on the ratio of misalignment to torque and on coupling geometry. The various forms of pressure distribution over the tooth surface have been presented. The analyses have also revealed the relative tangential displacement between the teeth for a complete revolution. This slip path differs significantly from that predicted by a rigid body analysis. An indication of maximum wear depth has been calculated for each coupling, assuming Archard's wear law applies. The wear depth varies axially along the tooth, being greatest at the ends. Several design and operating parameters thought to influence tooth wear have been investigated. The misalignment angle has the greatest effect on maximum wear depth whilst the torque has little influence. For a fixed diameter of coupling, wear appears to decrease for greater numbers of teeth. Minimising the diameter is the most effective way of designing for minimal wear. Equations have been presented in dimensionless form for parameters describing the wear, moment and pressure distribution. Experimental tests of modified spline couplings have supported the computational results and shown that effective lubrication is critical. Further experimental tests have allowed the fretting process and wear particle movement to be viewed. Software has been produced to simplify selection of the optimum spline geometry for an application.