Threshold stress intensities, KTH, below which no cracking will occur under stress-corrosion cracking or hydrogen-assisted cracking conditions, are discussed. A quantitative correlation, which suggests why KTH may exist, is evolved. The controlling mechanism is proposed to be a critical concentration of hydrogen, its propensity to form being governed by yield strength, initial concentration, state of stress and temperature variables. The general trends of all variables are reviewed for high strength AISI 4340, maraging and 9 Ni4 Co steels. Specifically it is proposed that Kapplied=KTHCeq=Ccr which means that the applied stress intensity equals the threshold when the equilibrium concentration of hydrogen at the crack tip just achieves the critical concentration. Based upon this concept, correlations are derived for elastic, plane-strain plastic and plane-stress plastic conditions.