SUMMARY This chapter deals with the effect of residual stresses due to welding on fatigue life under both constant-and variable-amplitude loadings. It is shown that residual stresses can significantly alter the S-N curve for welded joints. In particular, it is shown that, under constant-amplitude loading, residual stress affects the mean stress of small-amplitude cycles. Using simple hypotheses for the initial stress state and simple stress-strain behavior, the effect of mean stress for small amplitudes, using both the modified Goodman and the Gerber formulae, is investigated. Finally it is shown that consideration of residual stresses changes the S-N curve enough to cause substantial modification of variable-amplitude fatigue-life predictions by the commonly used Rayleigh approximation method.