In-situ deformation studies in a transmission electron microscope equipped with an environmental cell have shown that solute hydrogen increases the velocity of dislocations, decreases the stacking-fault energy, and increases the stability of edge character dislocations. Theoretical modeling has established that the hydrogen atmospheres formed at dislocations through the elastic interaction cause a change in the stress field of the dislocationhydrogen complex in such a manner as to reduce the interaction energy between it and other elastic obstacles. Consequently, solute hydrogen increases the mobility of dislocations, which will be localized to regions of high hydrogen concentration. On the basis of this material softening at the microscale, a solid mechanics analysis of the hydrogen solute interaction with material elastoplasticity demonstrates that localization of the deformation in the form of bands of intense shear can occur at the microscale. Thus, the present combined experimental and numerical/analytical results provide a clear explanation for the hydrogen-enhanced localized plasticity mechanism of hydrogen embrittlement.