Many R&D programs are being conducted worldwide to study and, in some cases, develop materials with the properties required by Gen IV reactor systems. These advanced reactor systems require materials having sufficient dimensional stability and mechanical properties to meet the anticipated near- and long-term service demands under extreme conditions. These conditions are most challenging in environments with high levels of irradiation damage coupled with requirements for corrosion resistance and severe mechanical loading conditions. Based on past experience, austenitic alloys based around 316SS compositions, 15% Cr-15% Ni-Ti, have not been able to meet these demanding conditions at elevated temperatures. Ferritic-martensitic steels and ferritic-martensitic ODS steels have been proposed for these conditions. However, long-term fast neutron irradiation results show that austenitic alloys of the 15%Cr-25%Ni-Ti-Nb class have excellent swelling resistance at temperatures up to 600°C and at doses larger than 150 dpa. A major reason for this excellent behaviour is the thermal-mechanical treatment of the alloy and the presence of both Ti and Nb to doubly stabilize the precipitate structure. This material could represent an effective option for claddings in SFR and other advanced reactor applications. For this reason, in consideration of 60-y-of-service requirement, predictions of long-term materials behaviour will be carried out taking into account ageing effects (from 500° to 700°C). A complete analysis aiming to characterize the microstructure of the as-received material, cold-worked at 20%, and also after thermal treatment is under way. Ageing tests as well as TEM and SEM analysis of the microstructural evolution as a function of aging condition will be performed. The evolution of some phases in the common nucleation and growth sites will be presented.