## 7 Conclusions  
Green hydrogen energy has emerged as one of the key solutions to the severe environmental challenges posed by conventional fossil fuels. With the rapid development of large-scale hydrogen infrastructure, fatigue behaviour under hydrogen environments has become a critical bottleneck affecting the safe and reliable operation of hydrogen-related equipment. In this review, recent progress on the fatigue behaviour of metallic materials in hydrogen environments is thoroughly discussed. The following conclusions can be drawn:  
* Classic micro-mechanisms such as HELP, HEDE, and AIDE have been developed to describe hydrogen embrittlement in metals. Multiscale interactions between hydrogen and microstructural features, such as dislocations and grain boundaries, are key contributors to the deterioration of strength-toughness synergy. While the transitions between different mechanisms have been explored, a unified mechanistic framework that can quantitatively distinguish dominant and secondary mechanisms across different regimes is still lacking. * Advancements in experimental techniques, including high-pressure hydrogen loading-fracture systems, permeation testing, and in situ TEM/AFM, have enabled real-time observation of hydrogen accumulation at crack tips, grain boundary decohesion, and hydride formation. Most studies confirm that hydrogen significantly accelerates fatigue crack growth in steels, leading to a pronounced reduction in fatigue life. * Various modelling approaches have been developed to predict fatigue life in hydrogen environments, including empirical formulas, physics-based models, finite element simulations, and machine learning techniques. Each method has its own advantages and limitations: empirical models enable quick estimation but lack mechanistic insight; physics-based and numerical models offer better physical fidelity but rely heavily on microstructural parameters and computational resources; machine learning shows great potential in high-throughput prediction but is constrained by data quality and interpretability. * Multiple mitigation strategies have been proposed to extend the service life of components in hydrogen environments, including the development of low hydrogen-sensitivity alloys, surface coatings/diffusion layers, electrochemical protection, hydrogen de-trapping via heat treatment, stress-relief structural design, and real-timemonitoring. Nevertheless, the development of novel hydrogen-resistant materials and protective systems remains a major challenge. Looking forward, further research is needed in the following key directions: (1) Establishing a unified multiscale mechanistic framework linking hydrogen embrittlement and fatigue. (2) Building standardised databases that cover high-temperature, high-pressure, and cyclic loading conditions. (3) Developing AI-driven digital twin frameworks for real-time monitoring and closed-loop prediction. (4) Creating risk-based life assessment and maintenance optimisation strategies. Advancing these areas will be essential to ensuring the long-term safe operation of hydrogen infrastructure and equipment. Conceptualization, S.Y., D.M., A.M.P.D.J. and P.N. ; Methodology, S.Y., P.N. and A.M.P.D.J. ; Validation, S.Y., D.M. and Y.S. ; Formal analysis, S.Y., P.N. and Y.S. ; Resources, S.Y., D.M., P.N., A.M.P.D.J. and Y.S. ; Writing--original draft, S.Y. and P.N. ; Writing--review and editing. S.Y., D.M., A.M.P.D.J. and P.N. ; Funding acquisition, S.Y., D.M. and A.M.P.D.J. All authors have read and agreed to the published version of the manuscript. The authors gratefully acknowledge the fundings from Project ATE: Agenda para a Transicao Energetica (02/C05-i01.02/2022.PC644914747-00000023), cofinanced by Plano de Recuperacao e Resiliencia (PRR), Republica Portuguesa, through NextGeneration EU.