A heat exchanger for a first fluid and a second fluid, the heat exchanger comprising: a plurality of interconnected separator members , the separator members respectively including a first surface and an opposite second surface ; the separator members respectively including an array of wave features that make the first surface and the second surface uneven and that make the first surface inverse to the second surface , characterised in that the wave features being spaced apart across the respective separator member along a first axis and a second axis , the first axis and the second axis being orthogonal, the wave features defining a plurality of apexes and a plurality of recesses with the plurality of apexes being surrounded by one or more of the plurality of recesses , the first surface being inversely contoured relative to the second surface with the apex of the first surface defining a recess of the second surface ; the separator members being stacked and disposed in an alternating arrangement with the first surfaces of adjacent separator members facing each other, and attached at the respective wave features , and with the second surfaces of adjacent separator members facing each other and attached at the respective wave features , wherein opposing wave features project toward each other and attach together with the apexes of the first surfaces of adjacent separator members being attached and the apexes of the second surfaces of adjacent separator members being attached; a plurality of first flow passages defined between respective first surfaces of the separator members , the first flow passages configured to direct flow of the first fluid therethrough; and a plurality of second flow passages defined between respective second surfaces of the separator members , the second flow passages configured to direct flow of the second fluid therethrough, the second fluid and the first fluid being configured to exchange heat through the separator members . a plurality of interconnected separator members , the separator members respectively including a first surface and an opposite second surface ; the separator members respectively including an array of wave features that make the first surface and the second surface uneven and that make the first surface inverse to the second surface , characterised in that the wave features being spaced apart across the respective separator member along a first axis and a second axis , the first axis and the second axis being orthogonal, the wave features defining a plurality of apexes and a plurality of recesses with the plurality of apexes being surrounded by one or more of the plurality of recesses , the first surface being inversely contoured relative to the second surface with the apex of the first surface defining a recess of the second surface ; the separator members being stacked and disposed in an alternating arrangement with the first surfaces of adjacent separator members facing each other, and attached at the respective wave features , and with the second surfaces of adjacent separator members facing each other and attached at the respective wave features , wherein opposing wave features project toward each other and attach together with the apexes of the first surfaces of adjacent separator members being attached and the apexes of the second surfaces of adjacent separator members being attached; a plurality of first flow passages defined between respective first surfaces of the separator members , the first flow passages configured to direct flow of the first fluid therethrough; and a plurality of second flow passages defined between respective second surfaces of the separator members , the second flow passages configured to direct flow of the second fluid therethrough, the second fluid and the first fluid being configured to exchange heat through the separator members . <EOS>