An overview is provided of our current understanding of SCC of Mg alloys. TGSCC is caused by an interaction of 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 prior research indicates that many Mg alloys have a threshold stress for stress corrosion cracking of the order of half the yield stress in common environments including high-purity water.SCC fracture surface for AZ91 showing fracture of a β-particle ahead of the min crack