The recent advances in reactive extrusion technology, involving reactive modifi cation of polymers and/or reactive blending to form graft or block copolymer compatibilized blends in an extruder, have resulted in several successful commercial blends of otherwise highly immiscible polymer pairs, such as polyamide/olefi nic elastomer, polyamide/polyolefi n and polyamide/PPE blends. In many cases, the reactive modifi cation of the base polymers and the subsequent reactive blending can be combined into a one-step, sequential operation in a twin-screw extruder, making this an economically attractive process. Reactive compatibilization is still an emerging technology for the development of useful blends from otherwise highly immiscible polymers, and particularly those of crystalline polymers. Specifi c examples of reactive modifi cation and reactive blending will be discussed later in individual cases. Further details on compatibilization strategies can be found in chapters 4 Interphase and Compatibilization by Addition of a Comptabilizer and 5 Reactive Comptabilization of Polymer Blends of this book.