This chapter begins by introducing the conditions necessary for homogeneous versus heterogeneous nucleation of the solid phase from a liquid. Various mechanisms of solid front growth from the liquid state are analyzed, starting with the simplified case of pure metals and progressing to the compositional complexities of alloys. The phenomenon of compositional undercooling and the unique microstructures it produces are discussed in detail. Casting technologies are then introduced to provide practical examples that relate back to the earlier theoretical discussions. Finally, key solidification defects—such as gas porosity, shrinkage porosity, hot tearing, and inclusions—are examined to underscore the critical importance of melt control throughout the solidification process.