The study of the relationship between polymer materials or composites, their structure and behavior on the one hand and their processing conditions on the other hand, is a multidisciplinary science. The heat transfer during processing of polymer and/or composites materials plays an important role. This chapter provides tools or data on polymer solidification during processing conditions. It first presents key experimental observations concerning crystallization in processing conditions, especially flow. Then, various modeling approaches based on these observations are reviewed. The chapter also reviews extensive theoretical developments on the crystallization of polymers subjected to non‐isothermal flow. Finally, an illustration of the process‐structure relationship is proposed through recent theories and the presented results and/or observations are dealing with the effect on crystallization kinetics of thermoplastic polymers, in order to quantify and discriminate, respectively, the thermal and flow effects on the material behavior.