An overview is provided of Mg SCC. Intergranular stress corrosion cracking (IGSCC) is typically caused by a continuous second phase along grain boundaries. The second phase causes microgalvanic corrosion of the adjacent Mg matrix. IGSCC is expected for all creep resistant Mg alloys in contact with water whenever creep resistance is produced by a continuous second phase along the grain boundaries. IGSCC can be avoided by appropriate Mg alloy design. Transganular SCC (TGSCC) is caused by an interaction of hydrogen (H) with the microstructure so a detailed study of H-trap interactions is needed in order to design alloys resistant to TGSCC. This understanding is urgently needed because Mg alloys are being increasingly used in load bearing applications; many common Mg alloys have a threshold stress for SCC of half the yield stress in common environments including high-purity water.