This research is concerned with the Al-Li-Cu-Mg alloy 8090. The development of aircraft structural materials, and in particular aluminium alloys, has been reviewed. The development of Al-Li alloys has also been reviewed and the effects of alloy additions and heat treatment on mechanical properties have been discussed. The microstructure of a plate, 25 mm in thickness, has been fully characterised in the solution treated, under-, peak- and over-aged condition after ageing at 190oC. Coarsening of the two major precipitation hardening phases, ' and S-phase, has been related to mechanical properties and fracture behaviour. A post solution-treatment stretch has been found to bring about a finer, more uniform distribution of S-phase which results in a more homogeneous slip distribution and improved mechanical properties. Room temperature ageing was found to occur due to the growth of ' precipitates. The dislocation networks introduced during the stretching operation swept away a proportion of quenched in vacancies, resulting in a more sluggish ageing response. A short room temperature pre-age caused the formation of a grain boundary PFZ and retarded S-phase growth in unstretched material. Elevated temperature soak and recovery tests were carried out on peak-aged tensile specimens. Strength decreased in both series of tests above 150oC, due to a progressive dissolution of ', which was accompanied by an increased ductility in tests at soak temperature. Unstretched material showed a lower amount of dynamic recovery and a reduction in work hardening rate at a higher temperature in comparison with material in the stretched condition. These effects are accounted for by the difference in the effective S-phase size. High cycle fatigue behaviour over the temperature range 288 K to 425 K has also been studied and again the unstretched material maintains its E.R. to a higher test temperature. (D75129/87)