Continuously rising commodity prices and increasing competition make it necessary to develop new and improve existing processing technologies. Thereby, physical foaming in extrusion represents an excellent way to save natural resources while maintaining high quality products. However, physical foaming is a very complex processing technology, whereas a lot of contradictions are present in the existing literature. Therefore, the individual steps of physical foaming in extrusion were systematically investigated in this thesis. Among other things, a measurement concept for the determination of the minimum required residence time for the formation of single-phase solution and a new dynamic measuring method for the determination of blowing agent solubility in the polymer melt were developed for this purpose. It could be shown that the shear rate and the pressure at the blowing agent injection point are essential for the formation of single-phase solution, whereas the mass temperature has little influence on it. The studies on phase separation in foam extrusion have shown that both the addition of passive nucleating agents and the increase of shear and strain rate in the die lead to a significant increase of degassing pressure. The increase of the degassing pressure results in a significant improvement of the foam morphology for low blowing agent concentrations (