Abstract: Hydrogen embrittlement （HE） is a critical materials-failure issue in hydrogen-containing environments encountered in modern chemical and energy-processing industries. Its intrinsic complexity spans a multiscale cascade—from atomic-level events to macroscopic fracture—and directly threatens the safety and service life of hydrogen-energy infrastructure and chemical-plant equipment. Yet a comprehensive, multiscale review that integrates the full spectrum of HE research in metallic materials remains lacking. This review adopts a multiscale framework to synthesise current understanding of HE phenomena and mechanisms and to identify future research priorities. At the macroscale, we examine how operational parameters such as hydrogen blending ratio, temperature, humidity and applied stress synergistically influence HE susceptibility, highlighting their nonlinear coupling in governing failure behaviour. At the mesoscale, we show how grain size, grain-boundary character and phase-interface morphology regulate hydrogen diffusion, segregation and crack initiation, demonstrating that mesostructural features dictate hydrogen distribution and preferred damage sites. At the microscale, we analyse the interactions between hydrogen atoms and lattice defects—including dislocations, grain boundaries, twin boundaries and nano-precipitates—to clarify the atomic origins of embrittlement through hydrogen trapping, transport and accumulation. Building on insights across these scales, we propose an integrated micro-atomic → meso-interface → macro-crack framework that links cross-scale phenomena to reveal the chain-transfer mechanism of hydrogen-induced failure under coupled multiphysical fields. This unified perspective establishes a continuous failure pathway from local hydrogen enrichment to mesostructural evolution and ultimately to macroscopic crack propagation. Grounded in this multiscale overview, the work provides essential theoretical guidance for the design and optimisation of hydrogen-resistant materials and for safety assessment and lifetime prediction of hydrogen-energy systems operating in hydrogen-bearing environments.