After a general introduction of industrial compounding, the focus will be on controlling and ensuring the actual composition of a compound in a typical continuous twin-screw-extrusion process. Usually, people just assume to have exactly the prescribed formulation (%) of ingredients in each and every pellet, well, let’s consider what aspects control this. We will look at dosing of individual feed streams and the typical time scale of fluctuations that may occur. Next we have the time scale of back-mixing inside a continuous extruder, leveling out such fluctuations to a certain extent. And finally there is the time scale of post-blending of solidified and cut pellets (purposely or ‘for free’): silo’s, packaging, customer handling. What can we say about ‘in-pellet’ and ‘pellet-to-pellet’ homogeneity? How does a fluctuation in the feed streams propagate all the way to the finished pellets and what is the true composition when molding smaller or bigger articles finally. How to measure composition and on what scale? Should we sample the output frequently or rather control the input parameters, thus saving on endless analyses? Lots of questions, very relevant for industrial compounding, but no easy answers. So let me present my angle to this matter and illustrate with practical examples. Jos Janssen earned his PhD at Eindhoven University with Prof. Han Meijer in 1993 on a topic in fluid mechanics (droplet breakup and coalescence). Since then he has been working for 20 years in the plastics industry, first at GE Plastics (BoZ NL and Mt.Vernon IN, USA) and since 2000 at DSM (Geleen NL). His field has been mostly compounding technology, acting as an intermediate between product developers (who come up with new formulations in the lab) and large scale manufacturing (designing and optimizing production lines across the globe).