The present manuscript presents an analysis of flow and transport phenomena in microstructure reactors, their influence on the behavior of the reactors and the major interest and areas of application for the use of microtechnology in ptocess engineering. Hydrodynamic conditions and space-times in the numerous channèls are accurately controlled by appropriate design of the reactor. The residence time distribution, although narrow in a single microchannel, is significantly dispersed in the reactor due to flow singularities. Nevertheless, microstructured reactors are suited to periodic operation at frequencies higher than 1 Hz. The thermal behavior, characterized by significant axial conduction, leads to spatial isothermicity in metallic reactors and requires the use of poody conducting ni'aterials to maintain temperature gradients. Under transient conditions, following a step change in fluid temperatures; the reactor temperature variation can be represented by two characteristic times, one internal, the other external. In mass transfer, an axial diffusion effect can predominate with respect to convection, which increases the Sherwood number and back-mixing. Nevertheless, the determination of kinetic parameters being accurate for the low Damkohler numbers attainable, microchannelreactors provide favorable conditions for the study of rapid, reactions. A synthesis of the above-mentioned points indicates several advantages for microstructured reactors, such as a significant energy saving induced by the structuring of the,flow in' parallel channels. Intensification of heat exchange in microchanne1 dimensions limits therisks of thermal runaway by removing heat generatedhy exothermic reactions and allows miniaturization of production units without loss in productivity. Finally, lleat and mass transfer by conduction and diffusion are favored with respect to reacticin and hence microstructured reactors are well suited to kinetic measurement and to development of new processes limited by heat and mass transfer.