Heat transfer during flow boiling in minichannels and microchannels is intimately linked to the liquid-vapor phase distribution in the channels. The vapor phase exists as nucleating bubbles, dispersed bubbles, elongated bubbles, an annular core, or all vaporflow completely filling the channel. Similarly, the liquid can exist as bulk liquid, slags, thin film on the heated wall, or dispersed droplets. A detailed analysis of various structures, supported by high-speed video images, is carried out, and the effects on the associated heat transfer rates are discussed. This information provides insight into the flow boiling mechanisms and guidance for future research in this area.