The crack initiation site and the corresponding incubation time were determined as a function of notch radius in 4340 steel for both internally and externally supplied hydrogen. The source of hydrogen was found to affect both the crack nucleation site and the incubation time. Hydrogen cracking in cathodically charged 4340 steel initiated near the elastic-plastic boundary with incubation times which exhibited a linear dependence on notch radius. Hydrogen cracking in an aqueous solution initiated near the notch surface with incubation times which were relatively independent of notch radius. Short time diffusional flow models which include a stress dependent critical hydrogen concentration were found to predict incubation times reasonably for internally supplied hydrogen. Cherepanov's solution for the diffusion at the tip of a semi-infinite linear slit when applied in the context of a finite notch root radii problem was found to predict incubation times adequately for externally supplied hydrogen.