The microstructure and low-cycle fatigue properties of 20-25-Nb stainless steel in both the unirradiated and irradiated conditions have been studied over the temperature range 400 to 850C. The microstructure of the steel in the as heat-treated and aged, and neutron irradiated conditions is described, followed by a study of the effect of various test variables on the low-cycle fatigue endurance and fracture characteristics, together with some subsidiary work on tensile properties and fracture. A new type of elevated temperature low-cycle fatigue crack nucleation has been identified, and a quantitative model is presented to account for the increased low-cycle fatigue endurances associated with it. The influence of test variables on the phenomenon of square grain formation during low-cycle fatigue is studied, and interpreted in terms of a new model based on dislocation energetics. The low-cycle fatigue and tensile mechanical properties and fracture characteristics of the irradiated steel is next described and the deficiencies of the existing theory of elevated temperature embrittlement highlighted. The reasons for these deficiencies are discussed, and followed by a description of a qualitative model based on the existence of a damaged zone ahead of a graving fatigue crack, which it is believed offers an explanation of the observed mechanical and fracture characteristics of the irradiated steel.