This book provides insight into underlying basic theories, concepts, and methodologies in polymer phasing under the applied flow field. Polymer phasing covers the self-organization of phase structures of polymer mixtures and solutions through phase transitions. The phase transitions include liquid-liquid (phase separation) and liquid-solid (crystallization) transitions. This book describes the roles of flow on the flow-induced shift of phase diagram and transition points as well as the flow-induced cascade evolution of flow-rate dependent dissipative structures controlled by a balance of the applied mechanical energy against the thermodynamic potential for the growth of structures. The methodology covers in-depth real-time, in-situ, simultaneous investigation of light scattering, optical microscopy, birefringence, light transmittance, and shear and normal stress. The basic principles and concepts described in this book may be also applied in general to soft matters. This book is an excellent resource for graduate students, academic researchers, and researchers in industries.