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Hydrogen-assisted cracking (HAC) or hydrogen embrittlement (HE) are the most commonly used terms to describe a time-dependant failure process, characterized by mechanical properties degradation, mainly ductility reduction, and a change in the fracture mode. With almost no direct techniques for observing atomic-scale events at crack tips in bulk specimens, HE/HAC mechanisms are deduced mainly from fractography, microscopic investigation of microstructure changes, surface-science observations, and atomistic or continuum modeling. This paper addresses to HE/HAC mechanisms into two different systems; hydride-forming and non-hydride forming materials. As a representative example of the hydride-forming systems, the discussion focuses on the hydrogen-induced second phase formation (e.g. hydrides) phenomena in titanium based alloys. Due to the large differences in the behavior of hydrogen in α and β phases of titanium, the susceptibility of titanium-based alloys to the various forms and conditions of hydrogen embrittlement can vary markedly. The microstructural changes and hydrogen-induced second phase formation due to exposure at various charging conditions, as well as the difference in hydrogen absorption/desorption behavior as a function of the prior microstructure of titanium alloys, are highlighted. In non-hydride forming materials, where as representative examples stainless steels are chosen, the paper concentrates on the qualitatively same phenomena of hydrogen-induced second phase embrittlement. The phase transitions related to hydrogen-induced cracking, the hydrogen related failure sequences and fracture modes in austenitic and supermartensitic stainless steels and a potential modeling of hydrogen-assisted cracking in these structural metallic materials are discussed.