This paper presents results from a substantial investigation of residual stresses and defects associated with single pass and double pass friction stir welds in 5083-H321 and 5383-H321 aluminium. alloys. The residual stress part of the paper summarises data on their as-welded magnitude and plate-to-plate variation, together with their modification during applied bending fatigue loading corresponding to cyclic lives of 10(5) and 10(7) cycles. Results indicate fairly low initial peak tensile stresses both parallel with, and perpendicular to, the weld run. Peak tensile stresses occur just outside the tool shoulder with values typically in the range 0-30 Mpa. Peak compressive stresses have much higher magnitudes (typically in the range -50 MPa to -140 MPa) and occur at distances of up to 40 mm. from the weld centreline. Significant plate-to-plate variability in residual stress magnitudes exists, and fatigue cycling can raise peak tensile stresses by as much as a factor of four (to around 80 MPa). This has significant potential influence on fatigue life prediction. The paper also presents data on the occurrence of partial-fusion defects (PFD's or so-called 'kissing bonds' or 'onion-skin' defects) as a function of tool travel speed (in the range 80-200 mm/min), and their influence on fatigue life. Results indicate that PFD's can sometimes be associated with crack initiation, but that their major effect is more likely to appear when levels of plastic deformation are high, i.e. during relatively fast fatigue crack growth or during fast fracture.