We investigate hydrogen-induced crack initiation and subcritical crack propagation in high strength steels by addressing hydrogen transport and interaction with material deformation. Our experiments demonstrate that hydrogen reduces the fracture resistance of single-notched bend specimens of a lath martensitic steel while changing the morphology of the fracture surfaces from microvoid coalescence in the absence of hydrogen to a mixture of “quasi-cleavage” and intergranular. We modeled the onset of fracture in this steel by assuming that failure occurs by dislocation pile-ups against high angle grain boundaries whose strength is assumed to be reduced by hydrogen. The model reproduces the experimentally measured...