This article provides a brief outline of the state of the art of reactions using microreactors with special emphasis on the enhancement of product selectivity. The efficient micromixing possible with a short diffusion path increases the product selectivity of competitive parallel reactions and competitive consecutive reactions. This concept has been expanded to control of the molecular weight and molecular weight distribution in carbocationic polymerization. The efficient heat transfer, based on high surface to volume ratios, allows precise temperature control and is also effective for the control of highly exothermic reactions, such as free radical polymerization.