This chapter is a summary of recent advances in microreactors, which have become increasingly important in process development and chemical research. Microstructured chemical reactors are characterized by dimensions in the submillimeter range and, thus. surface-to-volume ratios are roughly two orders of magnitude greater than those of conventional reactors. The fundamentals of design and operation of microreactors are explained, and the necessary heat and mass transfer considerations are presented. Various designs are discussed, their key features are illustrated, and examples of successful applications are given. Emphasis is placed on the methods of introducing a catalyst into a microstructured reactor (MSR).