Mixing in polymerization processes is an extremely important issue as uneven mixing inevitably leads to the synthesis of polymer with undesired characteristics. Ontheotherhand, microeaction technology has enabled the development of extremely efficient micromixers which, within typical few milli seconds, 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, polymerswithimprovedcontrolovertheirmolecular 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 underless stringent reaction conditions.