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Abstract Microchannels are being considered in many advanced heat transfer applications including automotive and stationary fuel cells as well as electronics cooling. However, there are a number of fundamental issues from the heat transfer and fluid mechanics perspectives that still remain unresolved. The present work focuses on obtaining the fundamental heat transfer data and two-phase flow patterns present during flow boiling in microchannels. An experimental investigation is performed for flow boiling using water in six parallel, horizontal microchannels with a hydraulic diameter of 207 μm. The ranges of parameters are: mass flux from 157 to 1782 kg/m2s, heat flux from 5 to 930 kW/m2, inlet temperature of 22°C, quality from sub-cooled to 1.0, and atmospheric pressure at the exit. The corresponding single-phase, all-liquid flow Reynolds number range at the saturation conditions is from 116 to 1318. The measured single-phase, adiabatic pressure drop agreed with the conventional theory within the experimental error. The experimental single-phase Nusselt number was found to be between the constant heat flux and the constant wall temperature boundary conditions, corresponding to NuH and NuT respectively. The flow visualization demonstrates that the flow reversal condition in parallel flow channels is due to bubble nucleation followed by its rapid growth. In addition, the dry-out condition is observed, showing a change in the contact angles of the liquid-vapor interface. The local flow boiling heat transfer coefficient exhibits a decreasing trend with increasing quality. A comparison with the nucleate boiling dominant regime of a flow boiling correlation shows good agreement, except for the large peak in two-phase heat transfer coefficient observed at the onset of nucleate boiling.