The question of whether the monovacancy or the self-interstitial is the point defect responsible for impurity diffusion is reviewed in the light of buried marker diffusion experiments and irradiation-enhanced diffusion data. It is concluded that coupled vacancy-impurity diffusion is the preferred interpretation, but no definite conclusions can be drawn. However, the role of the self-interstitial in diffusion is proposed through a model which assumes that during oxidation, exchanges can take place between vacancies, self-interstitials, impurity interstitials, and substitutional impurities. By assuming a partial interstitialcy diffusion model it is shown that ?10-20% of the impurity flux during nonoxidizing diffusion conditions is due to impurity interstitials displaced by self-interstitials in equilibrium in the silicon. Calculations of impurity interstitial formation energies are consistent with the degree of oxidation enhanced diffusion measured for boron, phosphorus, and arsenic.