Theories of EL2 Defects.

The experimental situation with regard to the properties of the EL2 defect is now so rich that there is no microscopic model of EL2 which is not in conflict with one or more experiments, at least, as those experiments are now interpreted. We shall review the situation with regard to the theoretical predictions of the two most satisfactory current models, the isolated antisite and the antisite- interstitial pair, identifying what we see as obvious possibilities for reinterpretation of the experiments and also what we regard as more fundamental conflicts.