Predicting fatigue performance in the presence of residual stress fields remains daunting, particularly for welded structures, with documents such as BS7910:2013 giving generalised guidance on the likely values of residual stress in structures subject to post-weld heat treatment (section 126.96.36.199) in an enclosed furnace with temperature ranges between 550ºC to 620ºC. The guidance further notes, however, that where local post-weld heat treatment is carried out no general recommendations can be given and conservative assumptions should be made. Significant advances have been made in full-field measurements of residual stresses using sophisticated 3D synchrotron X-ray and neutron diffraction techniques, along with automated stages allowing precise location of measurement points and software-driven data analysis. At certain facilities, including the Institut Laue-Langevin (ILL) and the European Synchrotron Radiation Facility (ESRF) in Grenoble, France it is also possible to apply fatigue loading in-situ on a beamline whilst making residual stress measurements. It is then possible, for particular critical engineering applications, to measure 3D residual stresses and use them to calibrate detailed finite element models of complex welded (or other) structures to produce estimates of fatigue life. The large variability among residual stress levels in nominally similar welds and structural geometries, often arising from relatively minor variations in manufacturing, fabrication, heat treatment and service loading necessitate linking life prediction exercises to condition monitoring in service.