An additively manufactured heat exchanger , comprising: a microtube array comprising a plurality of microtubes forming a substantially parallel array and extending from a base plate , the plurality of microtubes comprising a first plurality of microtubes and a second plurality of microtubes; and a header comprising a plurality of header sections, the plurality of header sections comprising: a first header section integrated with the first plurality of microtubes at the base plate and configured to direct a first fluid through the first plurality of microtubes so as to exchange heat with an external fluid; and a second header section integrated with the second plurality of microtubes at the base plate and configured to direct a second fluid through the second plurality of microtubes so as to exchange heat with the external fluid; characterised in that the heat exchanger further comprises: a fin disposed between each adjacent microtube in at least one column of microtubes, the fin starting from the base plate to a surface of the microtubes; wherein the microtube array , the base plate , the fin , and the header are three-dimensionally (3-D) printed together to form a continuous body. a microtube array comprising a plurality of microtubes forming a substantially parallel array and extending from a base plate , the plurality of microtubes comprising a first plurality of microtubes and a second plurality of microtubes; and a header comprising a plurality of header sections, the plurality of header sections comprising: a first header section integrated with the first plurality of microtubes at the base plate and configured to direct a first fluid through the first plurality of microtubes so as to exchange heat with an external fluid; and a second header section integrated with the second plurality of microtubes at the base plate and configured to direct a second fluid through the second plurality of microtubes so as to exchange heat with the external fluid; characterised in that the heat exchanger further comprises: a fin disposed between each adjacent microtube in at least one column of microtubes, the fin starting from the base plate to a surface of the microtubes; wherein the microtube array , the base plate , the fin , and the header are three-dimensionally (3-D) printed together to form a continuous body. a first header section integrated with the first plurality of microtubes at the base plate and configured to direct a first fluid through the first plurality of microtubes so as to exchange heat with an external fluid; and a second header section integrated with the second plurality of microtubes at the base plate and configured to direct a second fluid through the second plurality of microtubes so as to exchange heat with the external fluid; characterised in that the heat exchanger further comprises: a fin disposed between each adjacent microtube in at least one column of microtubes, the fin starting from the base plate to a surface of the microtubes; wherein the microtube array , the base plate , the fin , and the header are three-dimensionally (3-D) printed together to form a continuous body. a fin disposed between each adjacent microtube in at least one column of microtubes, the fin starting from the base plate to a surface of the microtubes; wherein the microtube array , the base plate , the fin , and the header are three-dimensionally (3-D) printed together to form a continuous body. <EOS>