Polylactides (PLA) are processed in single or multiple screw extruders where thermal and mechanical stresses occur. These stresses can lead to melt degradation in terms of molecular weight loss. The susceptibility of PLA to degradation during processing is well known. Studying this degradation on extruders relative to varying key influencing factors such as temperature, shear rate or residence time is complicated due to the interdependency of these factors and varying screw geometry, melt temperature, pressure and melt velocity gradients along the screw channel(s). Therefore experiments with a rotational rheometer were performed. They allow a differentiated examination of shear and thermally induced degradation. The experiments show a considerable effect of the shear rate, temperature and residence time on melt degradation. Mathematical models describe the non-linear degradation mechanisms found in the rheometer experiments. An approach to transfer the rheometer results to a single screw plasticizing extruder (SSE) is presented. It shows a good qualitative agreement. Screw design and oper-ation point of the extruder are found to have a significant influence on the degradation of the processed PLA. During processing in a SSE a wide distribution of processing histories for different particles in the screw channel will occur due to longitudinal and transversal mixing of the melt. Thus a distribution of residence times, thermal and shear stresses has to be taken into account when using the models derived from rheometer experiments. Results of this model are presented.