The effect of friction stir welding (FSW) on the toughness properties of two aerospace aluminium alloys has been investigated. Two typical aerospace alloys, high strength AA7449 and medium strength AA6013 have been studied in detail. The mechanical properties have been characterised via hardness testing, toughness testing and tensile testing incorporating strain analysis via digital image correlation. A notched 5ar test has been used to produce a profile of toughness across each of the welds and, in AA7449, through the depth of the welded plate. Each fracture surface was examined via FEGSEM to determine the mode of fracture and the microstructure was characterised, via optical microscopy and FEGSEM, such that the microstructural changes caused by FSW could be linked to the variations in toughness.