A ‘critical size’ defect, as defined in fracture mechanics calculations, is influenced by the fracture toughness and microstructure of the material in the vicinity of the crack and also by the level of local stress assisting or impeding crack development. It has been realized at the National NDT Centre for some time that a balanced assessment of crack significance requires not only detailed information on crack location, crack dimensions and crack nature, but also quantitative data on microstructural variations and on levels of residual stress. The present paper discusses Harwell's programme in recent years to develop an ultrasonic technique to assess residual stress.