A fluid-to-fluid heat exchanger matrix comprising: a first plate having a first surface and a second surface; a second plate having a first surface and a second surface, the second surface of the first plate opposing the first surface of the second plate to define a first flow channel; a third plate having a first surface opposing the second surface of the second plate to define a second flow channel; the first plate, the second plate and the third plate comprising a portion of a plate matrix, wherein the matrix has a first flow inlet and a first flow outlet in communication with at least one of the first flow channel, and a second flow inlet and a second flow outlet in communication with the second flow channel; and wherein the second surface of the first plate has a plurality of variable flow structures arranged in a first region and a second region, the first region having a variable flow structure density greater than a variable flow structure density of the second region, to control a velocity of a fluid passing over the first and second regions in the first flow channel, the second surface of the second plate having a plurality of variable flow structures arranged in a first region and a second region, the first region having a variable flow structure density greater than a variable flow structure density of the second region to control a velocity of a fluid passing over the first and second regions in the second flow channel; characterised in that the first region and the second region of the second surface of the first plate defining different areas of the plate matrix, in plan view, than the first region and the second region of the second surface of the second plate. a first plate having a first surface and a second surface; a second plate having a first surface and a second surface, the second surface of the first plate opposing the first surface of the second plate to define a first flow channel; a third plate having a first surface opposing the second surface of the second plate to define a second flow channel; the first plate, the second plate and the third plate comprising a portion of a plate matrix, wherein the matrix has a first flow inlet and a first flow outlet in communication with at least one of the first flow channel, and a second flow inlet and a second flow outlet in communication with the second flow channel; and wherein the second surface of the first plate has a plurality of variable flow structures arranged in a first region and a second region, the first region having a variable flow structure density greater than a variable flow structure density of the second region, to control a velocity of a fluid passing over the first and second regions in the first flow channel, the second surface of the second plate having a plurality of variable flow structures arranged in a first region and a second region, the first region having a variable flow structure density greater than a variable flow structure density of the second region to control a velocity of a fluid passing over the first and second regions in the second flow channel; characterised in that the first region and the second region of the second surface of the first plate defining different areas of the plate matrix, in plan view, than the first region and the second region of the second surface of the second plate.