Polylactide (PLA) is the most important bioplastic on the market with a permanent growth in production capacity. Due to its good mechanical properties, it is comparable to polystyrene. But in terms of foaming the melt strength and extensibility of commercial polylactide are rather low. Therefore such commercial grades often are chemically modified to induce crosslinking, chain extension or grafting by means of reactive extrusion on a twin-screw extruder. In this work different polylactide grades were used, to better understand the molecular characteristics (molecular weight, acid value, D-content) not only on the modification mechanism, but also on the foam properties. Additionally different modifiers (chemical as well as physical modifiers) were used. The most effective results were achieved with organic peroxide. With this modifier the molecular weight, the melt viscosity, the strength and the crystallization rate were improved. Through foam extrusion foams with a closed-cell structure and good mechanical properties were achieved. Furthermore, the modifier was found to be more effective than a commercial modifier.