A variety of gas-liquid microchannel reactors have been developed so far, using different contacting principles. Some devices utilize continuous-phase contacting (i.e., nondispersed separate phases with large specific interfaces). Among these are microstructured falling film, overlapping channel, and mesh reactors. Dispersed-phase contacting is obtained when one of the phases is interdispersed into the other phase. Regular flow patterns are provided by the segmented (Taylor) flow in a single microchannel or numbered-up versions such as the microbubble column; other flow patterns such as annular flow may be achieved as well. Foam microreactors utilize a moving rigid 3-D bubble network at high gas content. Miniaturized packed-bed microreactors follow the paths of classical engineering by enabling trickle-bed operation. Because of the often highly regular flow pattern, not obtained in conventional gas-liquid contactors, an understanding of the underlying hydrodynamics and heat and mass transfer is crucial for optimal performance of all types of gas-liquid microstructural reactors. Several examples are given, including film-thickness measurements, flow-pattern maps, determination of mass-transfer coefficients, residence-time distributions, scale-out issues, etc. Numerous applications demonstrate the improved performance of gas-liquid microreactors. Among these are fluorinations, chlorinations, hydrogenations, sulfonations, photo-oxidations, etc. Recently, the scope of reactions has been widened, since there is now the possibility to carry out gas-liquid-solid processes in the same microreactors as used for noncatalytic reactions because of the development of catalyst washcoats and other materials deposited onto microchannels. Some relevant examples are given for illustration.