A heat exchanger for thermally coupling a first fluid and second fluid is disclosed. The heat exchanger comprises plates comprising cores of thermally conductive graphite foam. A plurality of conduits for conveying the first fluid is formed in each core. Each plate further comprises thermally conductive barriers that sandwich the core, wherein the barriers are substantially impervious to r the first fluid and second fluid. Plates are stacked in a frame such that the frame and plates collectively define a plurality of channels for conveying the second fluid. Heat is exchanged between the primary fluid and the secondary fluid through the graphite-foam cores and barriers.Heat exchangers in accordance with the present invention can be lighter, have improved ratio of heat transfer surface density to heat exchanger volume, be lower cost, and/or be smaller for a given heat transfer capability than prior-art heat exchangers.