Thermal transport in microchannels is investigated to§better understand the fundamentals of single-phase§convection and flow boiling at the microscale.§Measurements of single-phase pressure drop and heat§transfer suggest that conventional Navier-Stokes§theory and carefully selected correlations developed§at larger scales are entirely capable of predicting§single-phase transport§characteristics in the microchannels. Both§computational fluid dynamics analysis and approximate§analytical models are then presented to study the§microscale convection-conduction conjugate problem§and to aid in the practical design and optimization§of microchannel heat sinks. Convective flow boiling§in microchannels is also experimentally§characterized. Analytical models are developed to§predict the onset of nucleate boiling and convective§boiling heat transfer coefficients in microchannels§over a wide range of flow conditions. In addition, a§non-intrusive diagnostic technique, infrared§micro-particle image velocimetry, is developed to§facilitate the measurement of flow fields within§silicon-based MEMS devices with micron-scale resolution.