The continued safe and reliable operation of plant invariably has to consider the assessment of defects in welded structural components. This requires some estimate of the residual stresses that have developed during the welding fabrication process. There is an increasing use of computational weld mechanics to evaluate residual stresses and this paper describes the development of guidelines for using finite element analysis, which are now incorporated into the EDF Energy R6 defect assessment procedure, to determine stresses for use in the procedure. Prior to use in assessment the predicted results should be validated by comparison with measurements and the guidelines provide advice on the differing standards of validation which may be applied including the manufacture and testing of validation mock-ups. This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.