Multiphase systems are material systems consisting of at least two components. This can be a material system with two polymers (blend production) or a polymer with fillers. The use of multipolymer material systems aims to improve properties through synergistic effects, for example, ‘positive’ material properties of both polymers mixed together should improve the final component properties. The use of fillers in a polymer matrix, on the other hand, is aimed at functionalization and improvement, especially of the mechanical properties of the components. These two concepts are presented in the following chapter.