The aim of the project is to investigate the influence of residual stresses on the high- cycle, variable amplitude fatigue behaviour of welded structures and to make fatigue-life predictions. Two structural steels with different yield strengths (St 37-2 and St 52-3) and containing high tensile residual stresses will be studied. Both steels are used commonly in heavy vehicles and bridges. Flat welded specimens with longitudinal stiffeners (non- load carrying) containing residual stresses will be produced. Different measuring methods will be both for as welded and for stress-relieved specimens. The results will be correlated with fatigue data collected for variable amplitude loading and different frequencies typical of the service conditions.