The design of solidification is of great technological importance, since mechanical properties of the solidified poly-crystalline metal strongly depend on the mechanism and the rate of solidification/cooling. This chapter begins with discussion on the mechanism and microstructural evolution followed by the rate of solidification and casting characteristics; here the rates of nucleation and the growth are related to grain size, strength, and micro-segregation. Both usual and exceptional solidification behaviors are highlighted with reference to specific alloys. Cast metallic structures are illustrated with reference to the freezing range, including the technique to overcome the coring problem. The cooling curve has been explained with reference to the superheat and the total solidification time. Heat transfer in solidification is described with reference to Fick’s second law, and a workable mathematical formula is presented to determine the mold constant and the total solidification time, thereof. Finally, Chvorinov’s rule is explained, taking into consideration both flat and curved surfaces. This chapter includes 20 worked numerical examples, 10 diagrams, 12 formulae/equations, 6 data tables, and 10 MCQs with answers given at the end of the book.