This paper reviews the current understanding of the mechanisms of stress corrosion cracking of pipeline steels. The similarities, the differences and the influencing factors are considered for the "high pH" stress corrosion cracking caused by a concentrated bicarbonate-carbonate solution, and for the "low pH" stress corrosion cracking due to a diluter solution. For high pH stress corrosion cracking, it is well accepted that the mechanism involves anodic dissolution for crack initiation and propagation. In contrast, it has been suggested that the low pH stress corrosion cracking is associated with the dissolution of the crack tip and sides, accompanied by the ingress of hydrogen into the pipeline steel. But the precise influence of hydrogen on the mechanism needs to be further studied. (C) 2003 Kluwer Academic Publishers.