In the production of extruded plastic foams of medium and high density there are basically two different ways of supplying the blowing agent: Physical and chemical foam extrusion. In physical foaming, the gas is injected into the extruder in a gaseous or supercritical state. In chemical foaming, the required foaming agent is provided by the decomposition of a chemical blowing agent in the extruder. The achievable structures of such polymer foams depends on many different factors such as various processing parameters and the composition of the used material formulation. In this thesis different material formulations were physically and chemically foamed to investigate the various influences of the matrix polymer, the type and amount of nucleating agent used and the amount of blowing agent. Furthermore, different round dies were used in the foaming experiments to investigate the influence of the shear and elongation deformation work introduced into the melt on the cell nucleation. The polymers and polymer blends used as matrix were rheologically characterized with respect to their shear and elongation behavior. For this purpose frequency sweep tests and creep and recovery tests on a cone/plate rheometer and extensional viscosity measurements using a SER module (Sentmanat Extensional Rheometer) were performed. In addition to these standard measurements, a novel online Elongation rheometer was used to characterize the polymer melts online at the extruder during the process. Using this measuring device, the shear and extensional viscosities of the extruded polymers were analyzed under process conditions. Furthermore, the influence of the used physical and the chemical blowing agents on the viscoelastic material behavior of the polymers were investigated. A comparison of the different rheological data with the achieved foam structures should lead to a better understanding of the relationships between the material behavior of the matrix polymer and the foam properties. The comparison of the foam structures produced by physical and chemical foam extrusion shows that findings from one foaming process cannot easily be transferred to the other. Although the physical foam extrusion is much more complex, it offers the user far more freedom to control the foam properties. Chemical foaming, on the other hand, offers a much simpler process which is far less sensitive to numerous influences, both in terms of material formulation and process control. Howerver, the possibilities for targeted control of the foam properties are therefore considerably smaller than with physical foaming.