A thin multi-passage cooler that produces an isothermal heating surface, high heat transfer coefficients, without producing an excessive pressure drop across the coolant passages. The cooler having a plurality of first passages in fluid communication with a plurality of second passages. A coolant flows through the second passages and then through first passages which are in close proximity to a heating source. The first and second passages are oriented and connected such that the direction of coolant flow in the first passages is opposite to the direction of flow in the second passages. To promote high heat transfer coefficients the passages are constructed to have large length to width ratios and small hydraulic diameters.