Direct observations of the effects of H on dislocation mobility, carried out using an environmental cell TEM technique, have shown that the presence of H in solid solution increases the dislocation velocities at constant stress for edge, screw, and mixed dislocations and for isolated dislocations and dislocations in tangles.'' This increased dislocation velocity due to H has been observed in fcc, bcc, and hcp systems and in relatively pure'' materials, in solid solution alloy, in precipitation strengthened alloys, and in gamma prime strengthened alloys. Hydrogen softening'' or increased dislocation mobility due to hydrogen has been shown to result from elastic shielding'' which minimizes the elastic interaction between dislocations and obstacles. In contrast to the microscopic observations, many stress-strain measurements show an increase in the measured stress for plastic deformation when H is used as an alloy element. There remain significant differences in observations of the effects of H on the flow stress of metals. The present paper discusses one aspect of this problem, the effects of H on shear localization, which may account for the discrepancies.