Two-phase flows in microchannel heat sinks can increase heat removal capabilities in micro electronic devices. Two-phase flows include air-water flow and steam-water flow. An important aspect related to two phase flows in microchannels is the study of boiling phenomena and its characteristics. Flow boiling in microchannels raises a fundamental question on the manner in which the small channel dimension affect the bubble dynamics. Numerical analysis of boiling in two-phase flows is carried out by using Gambit and Fluent. The fluid flow and heat transfer characteristics of two-phase microchannel heat sinks are also determined. A review is carried out on flow boiling in both minichannels and microchannels.