Microchannel heat sinks allow removal of dense heat loads from high-power electronic devices at modest chip temperature rises. Such heat sinks are produced primarily using conventional subtractive machining techniques or anisotropic chemical etching, which restricts the geometric features that can be produced. Owing to their layer-by-layer and direct-write approaches, additive manufacturing (AM) technologies enable more design-driven construction flexibility and offer improved geometric freedom. Various AM processes and materials are available, but their capability to produce features desirable for microchannel heat sinks has received limited assessment. Following a survey of commercially mature AM techniques, three technologies were identified as meeting the fabrication requirements for a microchannel heat sink application; direct metal laser sintering (DMLS) was identified as the most promising candidate for the subsequent work. Straight and manifold microchannel designs were produced with hydraulic diameters of 500 microm in an aluminum alloy (AlSi10Mg) to assess the predictability of their performance and the impact of the fabrication method. Results indicated that the nominal geometry is reproduced accurately enough to predict pressure drop based on conventional hydrodynamic theory, albeit with roughness-induced early transition to turbulence; however, the material properties are not known with sufficient accuracy to allow for a priori thermal design. The manifold microchannel heat sink demonstrated successful integration of performance-enhancing features into a monolithic structure. A novel permeable membrane microchannel heat sink design was proposed that leverages additional benefits of the fabrication process, and benchmarked against a second manifold microchannel heat sink design. A reduced-order model was developed to study performance trends of the new design, which was then fabricated and experimentally evaluated. The permeable membrane microchannel design achieves both improved thermal and hydraulic performance at a constant pumping power compared to a manifold microchannel heat sink, and the same thermal performance can be achieved with half of the pressure drop in the new design. This demonstration illustrates the abilities of additive manufacturing to produce and integrate otherwise impossible features that compare favorably to conventional heat sink designs.