Strain-enhanced corrosion mechanisms of SCC, which include film rupture, the mechano-chemical effect, slip step dissolution, and the bare crack tip, all must meet the following two fundamental conditions: (1) the mechanism must produce a crack which lengthens much faster than it widens, and (2) each increment of strain must cause sufficient crack advance by corrosion to produce another equal increment of strain. A theory based on these two conditions leads to the requirement that, for film-covered metals, the ratio of crack-tip corrosion rate to crack tip strain rate must exceed a critical value in the range of 2·10−7 cm to 10−5 cm, depending on the strain gradient ahead of the crack tip. It is also shown that for film-covered metals the crack-tip strain must be concentrated on a small fraction of the crack tip surface, for instance by planar slip or by deformation near a grain boundary. A comparison of the theory with data from the literature suggests that the theory may be useful in predicting susceptibility to SCC.