Microreactor technology is an important method of process intensification. Liquid-liquid slug flow capillary microreactor has been used to intensify the mass transfer limited and strongly exothermic reactions. In this type of reactor, either two liquids flow alternate to each other in a capillary or one liquid flows as a continuous flow while other flows in the form of enclosed drops (slugs) depending on the interfacial tension between two liquids and liquid adhesion with the solid walls. The enhanced mass transfer is due to the internal circulations within the slugs which arise due to shearing action between the slug axis and the capillary wall or continuous phase. The slug size and the intensity of internal circulations depend on the type of mixing element and physical properties of the liquids. The proper understanding of physical behaviour of fluids at the microscale is a challenging issue for the growing microreactor application demands. This paper highlights the hydrodynamic characteristics of the liquid-liquid slug flow capillary microreactor.  Experimental results on flow regime, slug size and particle image velocimetry along with corresponding complementary start-of-the-art CFD simulations are discussed in detail.