Mechanically fastened joints have proven to be weak links in an aircraft structure. Therefore, these joints require high quality design and manufacturing processes. This chapter focuses on the joint design optimisation. Glare provides the means to improve the joint design using advanced material design. The enhanced fatigue properties will demonstrate some design advantages. The following key characteristics need to be highlighted: load transfer, secondary bending, influence of biaxiality and curvature and influences of fastener installation. Secondary bending induced by the inherent eccentricities of joints introduces a non-linear bending stress in the joints. A fast and simple means to calculate the stresses in joints is provided in this chapter and it is shown by tests that the neutral line model for this purpose is accurate. A complex three-dimensional stress system exists around rivets, which is affected by the installation procedures. Improved riveting installation procedures, which control interference fits, can delay crack initiation by a factor of ten. The joint fatigue behaviour for Glare is described in three parts, i.e. crack initiation, crack growth and residual strength. Fatigue crack initiation can be predicted using the Fokker severity concept adapted for Glare. Stress levels in equally thick aluminium and Glare sheets show higher stresses for the aluminium layers in Glare. Fibre-bridging and bridging by the intact aluminium layers stop the crack created by the earlier crack initiation in Glare. Crack growth is significantly slower in Glare than in aluminium joints.