Complex polymer systems are combinations of different types of polymers which can exist as single or multi-phase systems. The polymers of the components are chosen according to criteria like: - costs, - processing performance, - mechanical properties, - thermal properties, etc.. One of the main reasons for combining polymers is effectively costs. Polymer combinations are cost effective since mixing an expensive material with a less expensive one, provides increased performance at a lower price. Polymers are also blended to combine the specific properties of different materials in one. Crystalline and amorphous materials are good combinations. Amorphous materials are transparent and have a better dimensional stability; crystalline materials are stiffer. The combination of a thermoplastic material with an elastomer provides high stiffness and good impact resistance. Combining polymer materials permits to create new material properties, without developing a new material.