Models of varying complexity are available for understanding the role of composition on the solidification behavior of engineering alloys. The choice of an appropriate model is dictated by the solidification behavior of the alloy. In some situations, seemingly complex alloys can be represented accurately using simple binary models when only one eutectic reaction occurs during solidification. In contrast, some alloys form two or more eutectic constituents during solidification and require the use of ternary or multicomponent models. In this article, several examples are given on the use of binary, ternary, and multicomponent solidification models for understanding and controlling the solidification behavior and resultant microstructure of engineering alloys of practical interest. Examples are applied to Ni-Cr-Mo-Gd alloys currently under development for spent nuclear fuel applications; a nickel base superalloy (IN718); and dissimilar welds between a super austenitic stainless steel (CN3MN) and nickel alloy filler metal (IN686). Examples are also provided on applying the results of the solidification models for understanding and controlling the hot cracking response of the alloys.