This study highlights the development of small-scale reactors, in the form of microstructures with microchannel networking. Microreactors have achieved an impressive reputation, regarding chemical synthesis ability and their applications in the engineering, pharmaceutical, and biological fields. This review elaborates on the fabrication, construction, and schematic fundamentals in the design of the microreactors and microchannels. The materials used in the fabrication or construction of the microreactors include silicon, polymer, and glass. A general review of the application of microreactors in medical, biological, and engineering fields is carried out and significant improvements in these areas are reported. Finally, we highlight the flow patterns, mixing, and scaling-up of multiphase microreactor developments, with emphasis on the more significant industrial applications.