The mechanisms for grain refinement of magnesium alloys by superheating have remained ambiguous since 1931. A model has been proposed on the basis of the recent understanding of the grain refinement of both high purity and commercial purity Mg–Al alloys. The model explains most of the experimental observations about superheating. Analysis of the grain size data obtained from different Mg–Al alloys as a function of the growth restriction factor with and without superheating provides good support to the model.