The vacancy mechanism of diffusion in metals and alloys is discussed and a review is given of the experimental evidence that supports it. The freezing-in problem and the theory of radiation-enhanced diffusion are treated from the point of view of the vacancy mechanism. Also, the ideas underlying the displacement production rate and the rate of vacancy annihilation are outlined. Finally, the results of theory are compared with experimental observations, particularly in regard to the temperature- and flux-dependence of radiationenhanced diffusion.