HARMONISE puts forward a holistic approach for studying the body of knowledge required to accomplish harmonization and standardization of methodologies, codes and standards as well as the assessment of nuclear reactor components. Departure from a prescriptive-based to a performance-based approach in nuclear regulatory regimes is to be examined under the prism of conformity with the safety objectives of innovative fission and fusion facilities. To this end, data related to fusion installations is expected to stem from the ITER safety demonstrations, whereas data pertinent to advanced fission designs – such as fast breeder reactors and SMRs – will be extracted from relevant EC-funded projects. The basis for HARMONISE activities will be the outcomes of relevant research and cooperation activities in standardization and nuclear safety considering also the lessons learnt from the stress tests performed in the EU. HARMONISE will examine issues related to qualification, standardization, V&V and licensing of fission and fusion installations, while taking into account stakeholder involvement. The benefits of adopting digital twins of nuclear installations during the design phase will be reviewed, while also identifying the cross-cutting activities that contribute to collaborative research efforts between fission and fusion. HARMONISE will address issues related to the preliminary safety assessments and licensing needs of innovative fission and fusion installations; risk-informed, performance-based approaches in licensing reviews and regulatory decision-making; harmonisation and standardisation on component assessments, methodologies, codes and standards and draw lessons from earlier experience in harmonisation efforts. HARMONISE findings will be disseminated to the nuclear safety regulators of EU MSs along with the State Nuclear Regulatory Inspectorate of Ukraine as material to be considered during safety verification and licensing of future fission and fusion installations.