Abstract Knowledge of the propagation rates of sub-critical cracks exposed to primary circuit coolant is essential for the use of defect assessment procedures for validation of PWR pressure vessel integrity. The results of research programmes conducted by the UKAEA to assess the conditions under which the PWR environment influences crack propagation of low alloy steels under cyclic and steady loading are described. The measured corrosion fatigue crack propagation rates for low and medium sulphur steels in good quality flowing water were well below those predicted by the ASME Section XI, Appendix A assessment curves, but higher rates were attainable for high sulphur steel or under adverse environmental conditions. No influence of material microstructure due to welding was observed in high flow water. No susceptibility to stress corrosion has been observed in bolt-loaded specimen tests on a range of parent steels and weldments. Cracking in slow strain rate tests was observed only at high potentials, such as were produced by the presence of dissolved oxygen, unless the specimen orientation was such as to maximise access of PWR coolant to sulphide inclusions in the steel. The implications of the data for operating plant and progress with the development of improved methods for assessing crack growth in operating plant are discussed.