The scale-up/-down of polymn. reactors has to deal with large viscosity changes during the process, addressing mass- and heat-transfer issues. A practical example on scale-up of styrene and Me methacrylate free radical bulk and soln. polymn. is presented. The main crit. parameters are mixing at mol. level (micromixing) and heat removal capacity. The operating parameters being kept const. are: reaction conditions (temp., pressure, chem.) and thus the reaction time. A pilot plant issued from scale-down of possible industrial sizes was developed to represent, at best, larger scales. The main parameter being scaled-up is the heat removal capacity, which has to be maintained const. among the different sizes. New concepts are adapted to dissipate the mixing energy where it is the most suitable, and the final step is the scale-up/-down strategy. Another issue addressed in this contribution is the need for in-line analytics that could operate at different plant scales and thus give important information for process control. Scale-up/-down strategy must include the whole process, not only the reaction stage but also what happens before, after, and simultaneously, i.e., upstream, downstream, and peripheral operations. Finally the measurable success of a scale-up/-down anal. could only be proved at industrial scale, where a good agreement between pilot- and larger scale should be obsd. The concepts in terms of transfer phenomena, analytics, sepn., product properties, feasibility, and economics should also be included in this anal.