This book analyzes the effect of hydrogen on the atomic-level interactions in metals, detailing the corresponding changes in the physical properties of crystal lattice defects, diffusion, and phase transformations in metallic materials as a result of hydrogen loading. It presents a novel derivation of the structure of stacking faults, the mobility of dislocations, and short-range atomic order in hydrogen-infused metallic alloys based on the change in the concentration of free electrons. It reviews the current hypotheses behind hydrogen embrittlement of iron-, nickel, and titanium-based...