Sensitivity of API-5L X42 and X70 line-pipe steels to hydrogen degradation of fracture properties has been studied under a variety of loading conditions. Hydrogen-enhanced crack growth under fatigue and static loading was evaluated in numerous hydrogen-rich gas mixtures. Under cyclic loading, hydrogen accelerated fatigue-crack growth, depending on frequency, stress ratio, stress-intensity range, and composition of the gas mixture in the environment. Under static load, subcritical-crack growth has been shown to occur in high-strength steels, in weld heataffected zones, and in steel heat treated to simulate local hard regions in pipeline steels, J-integral fracture-toughness experiments have shown that hydrogen reduces the fracture toughnessJlc, and, in steels that undergo subcritical-crack growth, reduces the tearing resistance, dJ/da. A relationship between hydrogen-accelerated fatigue-crack growth and hydrogen-induced reductions in fracture toughness is presented. The reduction or elimination of hydrogen-degradation effects by the presence of certain inhibitor gases, such as oxygen (O2), carbon monoxide (CO), or sulfur dioxide (SO2), also is discussed.