The detrimental effects of the H on the mechanical properties of the metals are known for more than a century. One of the most important degradation mechanisms is H embrittlement (HE). In this thesis, we examined a few famous proposed mechanisms in the field by performing careful atomistic simulations. Moreover, novel mechanisms which can be responsible for HE process in metals are demonstrated in this work. First, we used atomistic simulations to investigate the effects of segregated H on the behavior of cracks along various symmetric tilt grain boundaries in fcc Nickel. Mode I fracture behavior is then studied, examining the influence of H in altering the competition between dislocation emission (â ductileâ behavior) and cleavage fracture (â brittleâ behavior) for intergranular cracks. Simulations revealed that the embrittling effects of H atoms are limited. We examined the effect of H atoms on the nucleation of intergranular cracks in Ni. The theoretical strengths are $\sim$ 25 GPa and the yield strengths are $\sim$ 10 GPa, so that (i) the theoretical strength is always well above the yield strength, with or without H, and (ii) both strengths are far above the bulk plastic flow stress, $\sigma_y^B$ of Ni and Ni alloys. So H does not significantly facilitate nucleation of intergranular cracks. We performed simulations of the interactions between dislocations, H atoms, and vacancies to assess the viability of a recently-proposed mechanism for the formation of nanoscale voids in Fe-based steels in the presence of H. The effectiveness of annihilation/reduction processes is not reduced by the presence of H in the vacancy clusters because typical V-H cluster binding energies are much lower than the vacancy formation energy, except at very high H content in the cluster. Experimental observations of nanovoids on the fracture surfaces of steels must be due to as-yet undetermined processes. The possible strengthening effects of H atoms in metals at low temperature is examined via the solute strengthening (SS) theory. The results of the SS theory can explain recent experimental observations of strengthening of H-charged polycrystalline nickel at low temperature. Moreover, the possible softening/hardening effects of H atoms due to their interaction with the pre-existing with solutes are demonstrated and for the first time, a softening process in nickel alloys is shown. The effect of the H atoms in increasing the precipitate hardening in $\alpha$-Iron is also shown in this thesis. The direct molecular dynamics simulations of the bow out of an edge dislocation in H-free and H-charged samples reveals that the presence of H atoms decreases the magnitude of the bow out of the dislocation. The hardening effect of H on the interaction of dislocations and grain boundaries in nickel is also investigated in this thesis. To this end, we simulated the interaction of mixed and screw dislocations with the grain boundaries that have access to the slip planes in nickel. The presence of H atoms along the grain boundaries induces stress in the neighborhood of the grain boundary. These stress fields can repel/attract mixed dislocations while the screw dislocations are not interacting with them. The simulation of the interaction of the mixed dislocations with the H-free and H-charged GBs shows hardening due to the presence of H atoms. The simulations of the screw dislocations do not show significant hardening due to the presence of this stress fi