In practice almost all polymer-based products are mixtures of polymers and additives in the form of composites (polymers with additives) and/or blends (polymers with polymers). This means that mixing is prevalent to almost all polymer processes. Of, course mixing is even more relevant to processes that are dedicated to this task, such as twin-screw extruders which are the most common equipment used for compounding in industry. In general a crucial element to realizing the full potential of the additive or polymer blend is to achieve a high level of distribution and dispersion during compounding. The simulation of mixing in polymer processes is a field in its own right where there are still many challenges and unanswered questions requiring research. In this investigation the flow of a thermoplastic melt in a co-rotating twin screw extruder is simulated. The simulated flow is used to determine quantitative measures that are relevant to the physical mixing processes. The simulated results are compared with experiments where different material systems under different process conditions are investigated. These include miscible and immiscible polymer blends as well as polymers with particulate fillers. The comparison with the experimental results can be used to improve the simulation of mixing and as a first step to establishing a quantitative description of the mixing ability of the process.