This chapter deals with the effects of microfluidics on preparative chemistry processes. The main characteristic features of microstructured reactors (MSR) are their high surface area-to-volume ratio in the range from 10000 to 50000 m2/m3. Various parameters are considered when selecting a reactor for chemical reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature, the residence time required, and the mass and heat transfer characteristics of the reactor. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances. To benefit from the advantages related to the microstructured devices, the rate of reaction should be sufficiently high to get acceptable reactant conversions at space times in the range of a few minutes or seconds.