A unified theory of stress corrosion is propounded taking into account slip emergence, dissolution and repassivation at the tip of an advancing crack. Critical criteria are defined for crack propagation in terms of slip frequency and dissolution and repassivation current densities. It is hypothesised that the value of the ratio of β, the repassivation constant, to v, the frequency of discharge of dislocations at the crack tip, will be crucial in determining whether stress corrosion crack growth proceeds or not. Consideration of the equations developed suggests that the value of β/ν lies within the range 2·3 to 3·2. It is suggested that stress corrosion occurs in the region of a critical anodic dissolution overpotential at which certain specific relationships are satisfied with respect to the fundamental vibration frequencies of the crystal lattice as described by Planck's equation for the mean energy of an oscillator. The application of this concept led to equations of the predicted kind in which stress corrosion is manifest as a ‘mid-point’ phenomenon.