Hydrogen embrittlement (HE) is a critical issue in metallic materials, leading to catastrophic failures in hydrogen-containing systems like pipelines and storage tanks. This review discusses hydrogen diffusion behavior, HE mechanisms, and characterization techniques. HE occurs due to hydrogen interaction with metal defects, reducing mechanical properties such as ductility, toughness, and strength. Hydrogen can enter metals through surface adsorption, and its diffusion involves physisorption, chemisorption, and hydrogen uptake. In aqueous environments, hydrogen diffusion follows the Volmer–Tafel–Heyrovsky reaction mechanism, while in gaseous environments, it involves physisorption, chemisorption, and absorption. Hydrogen diffusion in metals is governed by interstitial and quantum mechanical tunnel diffusion. Interstitial sites in fcc, hcp, and bcc structures accommodate hydrogen, with O sites in fcc and hcp and T sites in bcc. Quantum mechanical tunnel transport allows diffusion at low temperatures, though it is less significant at higher temperatures. Hydrogen microprint technique (HMT) visualizes hydrogen diffusion in steel microstructures, while electrochemical and gaseous permeation tests quantify hydrogen permeation. Fick's laws describe hydrogen diffusion dynamics, and effective diffusion coefficients are calculated using various methods. Hydrogen traps, including vacancies, dislocations, and grain boundaries, influence hydrogen diffusion by trapping hydrogen and affecting permeability. Solute atoms, grain size, and microstructure significantly impact hydrogen diffusion, with smaller grains and certain alloying elements reducing diffusivity. The microstructure of steel, including phase boundaries and grain size, plays a crucial role in hydrogen permeation. The hydrogen-enhanced decohesion (HEDE) mechanism suggests hydrogen reduces cohesive strength across lattice planes, while other mechanisms involve hydrogen-induced crack propagation and hydrogen trapping. Understanding these mechanisms is essential for developing strategies to mitigate HE in hydrogen-containing systems.Hydrogen embrittlement (HE) is a critical issue in metallic materials, leading to catastrophic failures in hydrogen-containing systems like pipelines and storage tanks. This review discusses hydrogen diffusion behavior, HE mechanisms, and characterization techniques. HE occurs due to hydrogen interaction with metal defects, reducing mechanical properties such as ductility, toughness, and strength. Hydrogen can enter metals through surface adsorption, and its diffusion involves physisorption, chemisorption, and hydrogen uptake. In aqueous environments, hydrogen diffusion follows the Volmer–Tafel–Heyrovsky reaction mechanism, while in gaseous environments, it involves physisorption, chemisorption, and absorption. Hydrogen diffusion in metals is governed by interstitial and quantum mechanical tunnel diffusion. Interstitial sites in fcc, hcp, and bcc structures accommodate hydrogen, with O sites in fcc and hcp and T sites in bcc. Quantum mechanical tunnel transport allows diffusion at low temperatures, though it is less significant at higher temperatures. Hydrogen microprint technique (HMT) visualizes hydrogen diffusion in steel microstructures, while electrochemical and gaseous permeation tests quantify hydrogen permeation. Fick's laws describe hydrogen diffusion dynamics, and effective diffusion coefficients are calculated using various methods. Hydrogen traps, including vacancies, dislocations, and grain boundaries, influence hydrogen diffusion by trapping hydrogen and affecting permeability. Solute atoms, grain size, and microstructure significantly impact hydrogen diffusion, with smaller grains and certain alloying elements reducing diffusivity. The microstructure of steel, including phase boundaries and grain size, plays a crucial role in hydrogen permeation. The hydrogen-enhanced decohesion (HEDE) mechanism suggests hydrogen reduces cohesive strength across lattice planes, while other mechanisms involve hydrogen-induced crack propagation and hydrogen trapping. Understanding these mechanisms is essential for developing strategies to mitigate HE in hydrogen-containing systems.