This research incorporates an experimental and modeling approach to the nucleation of equiaxed crystals in a casting. In the model the crystals are assumed to nucleate from dendrite fragments, and therefore the model is considered to be more realistic than the semi-empirical heterogeneous nucleation models employed heretofore. This research plan includes: (1) the use of a transparent aqueous analogue to substantiate the basis of a model; (2) examination of laboratory scale metallic castings to see if the model can be extrapolated with justification; and (3) study of grain structures in continuously cast steel as an ultimate test of the model. %%% This program links a fundamental approach to real foundry practice by developing improved predictive modeling of the columnar to equiaxed grain transition and the equiaxed grain size and size distribution. This subject is critical to accurate modeling of the grain structure of castings.