This book explains some interesting phenomena in two-phase flow and heat transfer in microchannels. The microchannel technology is an emerging technology that finds many interesting applications in industry and academic research. Miniaturization of scientific and industrial components is the motivation to carry out the microchannel research. Microchannel technology offers many advantages in automotive heat transfer, in microelectronics for removal of high heat fluxes.The research in macro channels have been carried out for a long period now but still two-phase in macro and especially in microchannels is very complicated phenomena. As such the results found for macro scale channels are not directly applicable and can not be extrapolated to micro scale channels. This book gives some insight into main differences between a micro and macro scale channel and some governing phenomena in two-phase flow and heat transfer are highlighted.