Foamed polymers are used in many applications due to their outstanding characteristics such as their reduced weight and thermal conductivity. A promising substitute in many industrial applications are polypropylene foams. Unfortunately, their high potential is limited by the low melt strength and melt elasticity. This thesis is embedded in the project “innovative polymer upcycling”, which focuses on the up-cycling of polypropylene from post-consumer waste by long chain branching. As the foam industry is considered as an application sector, the focus of this thesis is on the foam production by foam extrusion of selected thermoplastics, such as polypropylene. Therefore, the optimal processing conditions and process parameters for a single screw extruder set-up were determined and preliminary studies of the foamability of polystyrene and low-density polyethylene were conducted. The die temperature had the greatest impact on the foam quality, followed by that of the screw speed and the type of chemical foaming agent. The original die of the single screw extruder was extended with an additional forming section for this reason. Rheological properties, such as high melt strength and pronounced strain hardening also had an influence on the produced foam. The optimal processing conditions for an extrusion foaming process were found and foams of various thermoplastic resins were produced. Long-chain branching of polypropylene improved its foamability. Additionally, a linear polypropylene was long-chain branched by reactive extrusion with peroxydicarbonate (PODIC) and foamed with a chemical blowing agent in one step. The produced foam was compared to a long chain branched polypropylene foam from a commercial high melt strength polypropylene and the foam of the linear polypropylene resin. The foaming with PODIC and a chemical blowing agent proved to be a promising method for the production of polypropylene foams from linear polypropylene resins.