Hydrogen embrittlement is the loss of strength and ductility in metals and alloys as a consequence of the absorption of hydrogen atoms by the metal. Hydrogen can be adsorbed in its molecular form on the surface; then, it dissociates to atoms and diffuses into the bulk. Hydrogen, being a small molecule, in some situations readily diffuses through the metal crystal structure, where it can accumulate and seriously reduce the ductility and load-bearing capacity. Hydrogen can find locations in many metal sites depending on the energy. Hydrogen is slightly larger than all of these interstitials; therefore, additional energy is necessary for a lattice to accommodate a hydrogen atom. High hydrogen concentrations are known to occur at grain boundaries, slip bands/dislocations, and around second-phase particles and other interfaces. Surface-science techniques show that high concentrations of hydrogen are present on surfaces. Inside the material, the hydrogen can be trapped by binding to impurities, structural defects, or microstructural entities such as carbides in alloys. This binding can be attributed to stress fields, temperature gradients, chemical potential gradients, physical trapping, or electrostatic forces (proton-defect interaction). The hydrogen traps are either reversible (dislocation, stacking fault) or irreversible (grain boundaries, nonmetallic inclusions, precipitates, or individual solute atoms). The hydrogen occupancy is crucial for a specific type of defect to operate in the fracture process. Fracture events most often proceed under nonequilibrium conditions. Trapping of hydrogen in various defects proceeds sequentially on the entry of hydrogen and during loading. The interactions are crucial for hydrogen embrittlement due to variations in hydrogen concentration around defects and alterations in the densities and structures of defects. Hydrogen atom diffusion through a metal is governed by the rate at which the atoms hop between interstitial lattice sites (classical jumping mechanism) and the extent to which they become transiently trapped at microstructural trap sites. The impact of trapping on diffusion will depend on the density and distribution of trap sites and the depth of the associated potential well. The driving force of diffusion is the concentration gradient of lattice hydrogen, but the loss of lattice hydrogen by diffusion is compensated locally by reversibly trapped hydrogen. Hydrogen is transported via dislocations to grain boundaries. It is evident that hydrogen is associated with dislocations, and, at appropriate dislocation velocities, it is transported by these mobile dislocations. Hydrogen interacts with dislocations to influence the mobility of dislocations and may increase the propensity for deformation twinning, although this appears to be more of an indirect than direct effect. Hydrogen weakens and eventually breaks the cohesive bonds of iron atoms, resulting in a loss of overall strength. Characterizing HE has led to several terms used to describe different forms of HE. Some terminologies are used based on the source of hydrogen and the type of damage. Hydrogen may affect one or more stages of fatigue failure and subsequently may affect each regime of fatigue differently, depending on the material, loading parameters, and details of the hydrogen-containing environment.