The current generation of nuclear reactors (Gen IIâIII) guarantees a stable supply of nuclear energy for the next 2â3 decades but additional development of nuclear technology is needed to reduce further potential accident risks and above all to increase its sustainability. The development of next generation (Gen IV) reactors, especially fast reactors, will greatly increase sustainability and mitigate long term impact of nuclear waste, when combined with concomitant fuel recycling facilities, firmly establishing the circular economy via closed fuel cycles. Europe has defined as its priorities three fast reactor designs: the sodium fast reactor (SFR) as reference technology, the lead (and leadâbismuth) fast reactor (LFR) and the gas fast reactor (GFR) as alternative technologies. In 2010, Europe, under the umbrella of the Sustainable Nuclear Energy Technology Platform (SNETP), launched the European Sustainable Nuclear Industrial Initiative (ESNII). The objective is to demonstrate Gen IV fast reactor technologies with closed fuel cycle, to harness European research and industrial capabilities to build advanced fast reactor prototypes and develop supporting infrastructure. Four prototypes or demonstrators are being designed: ASTRID for SFR, MYRRHA (ADS/LFR pilot plant) and ALFRED (demonstrator) for LFR and ALLEGRO for GFR. The operating conditions envisaged for these future systems are extremely demanding: high temperature, intense and prolonged irradiation, chemically aggressive environments. Therefore, the necessary nuclear materials will be exposed to very taxing conditions while simultaneously needing to respect increasingly high safety requirements. The performance of nuclear (structural and fuel) materials is therefore an essential aspect to make Gen IV reactors a reality.