Tests were conducted to compare the thermal performance of five types of wafer thin coolers; a double pass microchannel cooler, two types of single pass microchannel coolers, and two versions of an impingement cooler. The primary application of these devices is to remove heat from compact gallium arsenide diode wafers used in laser communications, but they can also be applied to a variety of other applications ranging from high density electronic packaging to hypersonic surfaces. The coolers were designed to absorb heat fluxes of over 100 W/cm2 with minimal surface temperature gradients. The coolers had a heat input area of 1 cm2, used water as the cooling fluid and had thicknesses ranging from 1 to 1.8 mm. One single pass microchannel cooler was made of beryllium oxide. The other four coolers were made of copper. A. special heat flux amplifier was built to obtain the high heat flux values with conventional heaters, and to provide instrumentation to determine temperature gradients. Typical results at 125 W/cm2 heat flux and 10 kg/hr coolant flow rate showed heat transfer coefficients and maximum surface temperature differences of 8 W/cm2/°C and 0.7°C for the double pass microchannel, 6.5 W/cm2/°C and 6.8°C for the copper single pass microchannel, 3.9 W/cm2/°C and 6.1°C for the beryllium oxide single pass micro-channel, 3.5 W/cm2/°C and 2.5°C for the first impingement cooler, and 5.3 W/cm2/°C and 2.4°C for the second impingement cooler.