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The fracture toughness is a key materials data requirement for practical defect assessments ofengineering plant. However, the apparent toughness or load to fracture can be affected by thehistory of loading. For example, prior loads due to proof pressure tests (PPT) or warm prestress(WPS), and sustained loading (cold creep) may all have an effect. Moderate levels ofappropriate PPT or WPS loading are generally considered to improve the subsequentapparent toughness properties, whereas, in the susceptible temperature and stress range, timedependentcrack growth can occur under sustained loading.In this paper, WPS data on ferritic steels are compared with the predictions of simplifiedmodels. The analyses validate forthcoming revised advice within the R6 defect assessmentprocedure. Furthermore, recent studies within the R6 development programme leading toupdated advice on the treatment of the PPT and sustained loads are also briefly described.