The effect of flow is of industrial relevance in melt extrusion and injection molding where temperature gradients and high deformation rates are encountered. This demands a better understanding of the flow dependence of the phase behavior in polymer blends since the morphology and properties of the finished product are influenced to great extent by the details of dynamics of the flowing melt. Experimentally both shear induced mixing and shear induced demixing phenomena have been observed under a variety of operational conditions. The primary objective of this book is to explain the phase behavior under industrially realistic deformation rates using a partially miscible SAN/PMMA blend as an example and to clarify the effect of shear flow on morphology of injection-molded sample. The reader will briefly learn the basic Thermodynamics of phase separation, Thermodynamics of polymer blends, and how to detect polymer miscibility by using Raman Spectroscopy and other techniques. The effect of shear flow on phase behavior of polymer blend is demonstrated theoretically and experimentally.