This paper presents details of fabrication and performance testing of prototype microchannel heat exchangers. The microchannel heat exchangers are being developed for advanced cooling and climate control applications, and are designed for heat loads of 100 W/cm2. Bulk and surface micromachining techniques are used to fabricate the test devices. Each heat exchanger section consists of over 150 microchannels etched in silicon substrates by either chemical etching or ion milling processes. The channels are 100-micrometers deep, 100-micrometers wide, and spaced 50- to 100-micrometers apart and connected with headers. Other heat exchangers have also been fabricated in copper and aluminum using machining and ion milling processes. Process steps involved photolithographic patterning, deposition of etch masks, ion or chemical etching, electrostatic bonding of the silicon to glass, insulator deposition, lamination of silicon to metals, application of thin heater coatings with busbars, and installation of the inlet/outlet hardware and valves. Recent hear exchangers have the silicon laminated to copper substrates. Performance testing focuses on determining the performance characteristics of the microchannel heat exchangers over a wide range of flow and heat transfer conditions. The working fluid for heat transfer is restricted to water or SUVA refrigerant HCFC-124 (R-124). Testing with water is run under single-phase conditions. The tests with R-124 are run under single-and two-phase flow conditions.