Two-phase forced convective flow in microchannels can be used as an effective means of cooling microelectronics. However, the number of studies on boiling in microchannels with dimensions smaller than 100 microns is limited. Particularly, the impact of small dimensions on bubble formation and the critical heat flux have received little attention. This work develops single and multi-channel silicon test devices with integrated heaters and thermometers, in an effort toward precisely determining the flow condition, the temperature distribution, and the heat transfer coefficients within microchannels. Rectangular channels with hydraulic diameter below 100 microns and varying aspect ratios were fabricated. The test devices have channel walls with widths below 350 microns, which minimizes solid conduction along the test section and reduces variations in the heat flux boundary condition. A semiconductor resistor strip measures the wall temperature distribution along the channel during phase change. A thermal resistance model has been developed to estimate the heat loss from the system. Down to hydraulic diameters as small as 25 microns, nucleation boiling under 5 °C of wall superheating was observed in plasma etched silicon microchannels.