Mixing in polymerization processes is an extremely important issue as uneven mixing inevitably leads to the synthesis of polymer with undesired characteristics. On the other hand, microeaction technology has enabled the development of extremely efficient micromixers which, within typical few milliseconds, allow mixing fluids at the microscale level. Recent developments in polymerization reaction engineering include the use of such micromixers to mix either initial reactants or reactive viscous solutions in multistep processes. Thus, polymers with improved control over their molecular weights and molecular weight distributions, chemical compositions and architectures can be synthesized. Micromixers can also be used to open new operating windows in which controlled polymerization can be performed faster or under less stringent reaction conditions.