The patterns forming during adiabatic gas-liquid flows in single microchannels are reviewed and the parameters influencing pattern transitions are discussed. Six major patterns were identified: the surface tension dominated bubbly and Taylor flows, the transitional churn and Taylor-annular flows, and the inertia dominated dispersed and annular flows. From the various parameters that have been studied in the literature, channel size, phase superficial velocities, liquid phase surface tension, wall wettability, and inlet conditions were found to affect the flow pattern formed while channel cross sectional geometry affected the patterns but to a lesser degree. Liquid viscosity and flow orientation with respect to gravity also seemed to play some role but the results were not conclusive. A universal flow regime map does not seem to exist and this is attributed to a lack of consistency in the inlets used in the various studies as well as to the effects of wall properties, such as wettability, contamination and roughness which are not usually varied systematically or reported. From the different flow regime maps suggested, those using U-GS-U-LS as coordinates represented better the transitions between patterns. (C) 2009 Elsevier Ltd. All rights reserved.