AbstractStress corrosion cracking (SCC) of Mg alloys is either intergranular (IGSCC) or transgranular (TGSCC). IGSCC is typically caused by a continuous second phase along the grain boundaries, which accelerates the corrosion of the adjacent Mg matrix by micro-galvanic corrosion. Thus IGSCC is expected in all such alloys, typical of most of the creep resistant alloys. All Mg alloys with continuous second phases are expected to be prone to IGSCC because each known second phase is less reactive than the matrix alpha-Mg; the degree of severity depends on the electrochemical properties of the second phase; these electrochemical properties need to be studied. IGSCC can be avoided by appropriate Mg alloy design. Of more interest TGSCC, the intrinsic form of SCC, is caused by an interaction of hydrogen with the microstructure so a study of H-trap interactions is needed in order to understand this damage mechanism and in order to design alloys resistant to TGSCC. This understanding is urgently needed if wrought alloys are to be used safely in service because prior research indicates that many Mg alloys have a threshold stress for SCC of about half the yield stress in common environments including high-purity water.