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This paper addresses heat rejection concepts intended to reject 100 W (or more) from the CPU of a desktop computer or server. A high-performance, air-cooled thermo-syphon heat rejection system is described that meets the goal. This concept is believed to represent the highest performance that has been reported for an air-cooled heat rejection system. A working fluid is boiled in a small chamber mounted on the CPU, and the vapor is condensed in the tubes of the remotely located air-cooled condenser. The air-cooled heat exchanger provides much higher performance than is obtainable with conventional "round-tube" technology. The boiling design is compared with other systems that have been proposed, e.g., liquid or two-phase cooling in micro-channels, and is shown to provide better performance. The heat exchange technology may also be applied to refrigerated cooling.