Molecular properties are reflected in the rheological and crystallization behavior. The first task is to design and use some well‐defined experiments that reveal all these features. Second, the experimental findings should be translated into mathematically formulated physical models, suited to be implemented in numerical codes for simulation of polymer shaping processes. Third, this modeling should be validated for a range of conditions including processing conditions. This chapter focuses on these three aspects and addresses general questions: How do the different structures observed relate to the flow conditions: deformation rates, stress, pressure, thermal conditions, material parameters, additives, and so on? and How can one model this on a continuum level so it is applicable in numerical codes for process simulation? The chapter also focuses on the effect of flow on crystallization of polymer melts.