Hydrogen embrittlement (HE) is a critical issue in metallic materials, leading to catastrophic failures in hydrogen-containing systems such as pipelines and storage tanks. The article reviews the mechanisms and characterization techniques of hydrogen diffusion and its effects on material properties. Hydrogen embrittlement is primarily caused by the interaction between metal defects and hydrogen, affecting ductility, toughness, and strength. Key mechanisms include hydrogen adsorption, diffusion, and interactions with intrinsic trapping sites like dislocations, voids, grain boundaries, and oxide/matrix interfaces. Common testing methods include tensile, double cantilever beam, bent beam, and fatigue tests. The entry of gaseous and aqueous hydrogen into metals involves surface adsorption, physisorption, chemisorption, and hydrogen uptake. Aqueous hydrogen diffusion follows the Volmer–Tafel–Heyrovsky reaction mechanism, while gaseous diffusion involves physisorption, chemisorption, absorption, recombination, and desorption. Hydrogen diffusion mechanisms include interstitial diffusion and quantum mechanical tunnel diffusion, with the latter being significant at lower temperatures. Characterization techniques for hydrogen diffusion include the hydrogen microprint technique (HMT) and hydrogen permeation tests (electrochemical and gaseous). HMT visualizes hydrogen diffusion through silver microparticle reduction, while permeation tests quantify hydrogen distribution and diffusion rates. The effective diffusion coefficient (D_eff) is calculated using various methods, and the impact of hydrogen traps on diffusion behavior is discussed. Factors affecting hydrogen diffusion include hydrogen trapping, solute-atom traps, grain size and boundaries, dislocations, and microstructure. Hydrogen traps can be reversible or irreversible, with reversible traps playing a more significant role in hydrogen-induced cracking. Dislocations act as weak hydrogen traps, and their presence can either reduce or accelerate hydrogen diffusion. Microstructure, such as grain size and phase interfaces, also influences hydrogen permeation characteristics. Proposed mechanisms of hydrogen embrittlement include the hydrogen-enhanced decohesion mechanism (HEDE), hydrogen pressure theory, and hydrogen-enhanced localized plasticity (HELP). HEDE weakens lattice bonds, hydrogen pressure theory explains cracking due to local pressure accumulation, and HELP involves hydrogen accumulation near crack tips, increasing dislocation mobility and local yield stress drop. Keywords: hydrogen embrittlement; hydrogen diffusion; damage mechanisms; mechanical propertiesHydrogen embrittlement (HE) is a critical issue in metallic materials, leading to catastrophic failures in hydrogen-containing systems such as pipelines and storage tanks. The article reviews the mechanisms and characterization techniques of hydrogen diffusion and its effects on material properties. Hydrogen embrittlement is primarily caused by the interaction between metal defects and hydrogen, affecting ductility, toughness, and strength. Key mechanisms include hydrogen adsorption, diffusion, and interactions with intrinsic trapping sites like dislocations, voids, grain boundaries, and oxide/matrix interfaces. Common testing methods include tensile, double cantilever beam, bent beam, and fatigue tests. The entry of gaseous and aqueous hydrogen into metals involves surface adsorption, physisorption, chemisorption, and hydrogen uptake. Aqueous hydrogen diffusion follows the Volmer–Tafel–Heyrovsky reaction mechanism, while gaseous diffusion involves physisorption, chemisorption, absorption, recombination, and desorption. Hydrogen diffusion mechanisms include interstitial diffusion and quantum mechanical tunnel diffusion, with the latter being significant at lower temperatures. Characterization techniques for hydrogen diffusion include the hydrogen microprint technique (HMT) and hydrogen permeation tests (electrochemical and gaseous). HMT visualizes hydrogen diffusion through silver microparticle reduction, while permeation tests quantify hydrogen distribution and diffusion rates. The effective diffusion coefficient (D_eff) is calculated using various methods, and the impact of hydrogen traps on diffusion behavior is discussed. Factors affecting hydrogen diffusion include hydrogen trapping, solute-atom traps, grain size and boundaries, dislocations, and microstructure. Hydrogen traps can be reversible or irreversible, with reversible traps playing a more significant role in hydrogen-induced cracking. Dislocations act as weak hydrogen traps, and their presence can either reduce or accelerate hydrogen diffusion. Microstructure, such as grain size and phase interfaces, also influences hydrogen permeation characteristics. Proposed mechanisms of hydrogen embrittlement include the hydrogen-enhanced decohesion mechanism (HEDE), hydrogen pressure theory, and hydrogen-enhanced localized plasticity (HELP). HEDE weakens lattice bonds, hydrogen pressure theory explains cracking due to local pressure accumulation, and HELP involves hydrogen accumulation near crack tips, increasing dislocation mobility and local yield stress drop. Keywords: hydrogen embrittlement; hydrogen diffusion; damage mechanisms; mechanical properties