Extensive work has been conducted over the last three years as part of a UK national project on the measurement of residual stress in components, in which over 250 individual X-ray diffraction tests were conducted to evaluate the sensitivity of XRD residual stress measurements to changes in the test set-up. These measurements provided information that enabled the uncertainty in the reported results to be evaluated for three different materials. The effect of changes in the test parameters is also shown, as is the variation in results obtained using different peak fitting routines.