The final properties of polymeric products are, to a great extent, determined by the internal structure that develops during practical processing. Both the flow and the thermo-mechanical history can influence the crystallization and orientation behavior of polymer melts and hence lead to different internal structures. Unlike simple molecules, polymer melts crystallize into a composite, two-phase structure that consists of an ordered crystalline region interspersed with an amorphous region in which the molecule chains are randomly arranged. Accurately predicting the evolution of such structures requires a deeper understanding of a polymer melt's crystallization and orientation behavior during practical processing; however, traditional theoretical and experimental methods are still difficult to apply to the investigation of such complex phenomena. Numerical modeling and simulation are highly effective methods that can be used to solve strongly nonlinear problems involving polymer processing and can further reveal the relevant mechanisms.