Over the last ten to fifteen years, microreaction technology has become of increased interest to both academics and industrialists for intensification of multiphase processes. Amongst the vast application possibilities, fast, highly exothermic and/or mass transfer-limited gas-liquid reactions benefit from process miniaturization. Recent studies of hydrodynamics and mass transfer in gas-liquid microreactors with closed and open microchannels, e.g., falling-film microreactors, are reviewed and compared. Special attention is paid to Taylor or slug flow in closed channels, as this regime seems to be most adapted for practical engineering applications.