Multiphase Flow in Polymer Processing focuses on dispersed and stratified multiphase flow in polymer processing. This book explores the rheological behavior of multiphase (or multicomponent) polymeric systems as they are involved in various fabrication operations. It also outlines the importance of the morphological states of multiphase polymeric systems to explain the systems, rheological behavior in the fluid state, and mechanical behavior in the solid state. This monograph consists of eight chapters divided into two parts. After discussing dispersed and stratified multiphase flow in polymer processing, it introduces the reader to the fundamentals of rheology. The following chapters focus on the rheological behavior of particulate-filled polymeric systems and heterogeneous polymeric systems; the phenomenon of droplet breakup in dispersed flow; and gas-charged polymeric systems. The role of the discrete phase (that is, solid particles, liquid droplets, gas bubbles) in determining the bulk rheological properties of the multiphase system is highlighted, along with some representative polymer processing operations (namely, fiber spinning and injection molding) of the multiphase (or multicomponent) polymeric systems. Coextrusion in cylindrical, rectangular, and annular dies is also considered. The final chapter is devoted to the phenomenon of interfacial instability in coextrusion. This text will be a useful resource for chemists, chemical engineers, and those in the polymer processing industry.