Analysis of hydrogen-stress field interactions have led to kinetic criteria for slow crack growth. Using both elastic and plastic stress fields under opening-mode loading, criteria for stage I, II, III growth are developed in terms of the pressure tensor gradient at the crack tip. It is proposed that stage I (stress-intensity dependent) growth kinetics are predominantly controlled by the elastic stress field while stage II (nearly stress-intensity independent) kinetics are controlled by the plastic stress field. Measurements of slow crack growth in cathodically-charged AISI 4340 steel verify the overall aspects of the correlation. Detailed measurement and analysis of the increase in crack-tip radius with increasing applied stress intensity have led to a proposed decrease in crack growth rate during stage II growth. Some experimental evidence corroborates this later hypothesis and is consistent with long range diffusional flow of hydrogen as the controlling mechanism for crack growth kinetics.