Due to the critical role of vapor confinement in establishing distinct flow and heat transfer characteristics in microchannels (as distinct from those in larger channels), the conditions under which such confinement occurs in microchannels are of great interest. It is shown in the present work that channel dimensions and flow properties alone, as proposed in past studies, are insufficient for determining confinement effects in microchannel boiling. Hence, a new criterion for physical confinement in microchannel flow boiling, termed the convective confinement number, that incorporates the effects of mass flux, as well as channel cross-sectional area and fluid properties, is proposed. This criterion helps determine the conditions under which a channel qualifies as a microchannel for two-phase flow, needing special treatment, and when a macroscale treatment is adequate. In addition, based on previous work by the authors, a new comprehensive flow regime map is developed for a wide range of experimental parameters and channel dimensions, along with quantitative transition criteria based on nondimensional boiling parameters. (C) 2010 Elsevier Ltd. All rights reserved.