A heat exchanger for exchanging heat between at least two fluids comprising walls (44, 46, 48) defining a plurality of passages, each of which passages extends between two termini (106, 108) of the heat exchanger and is defined by at least three of said walls (44, 46, 48) connected together, wherein each of said walls (44, 46, 48) of one of said passages is also one of said walls (44, 46, 48) of an adjacent one of said passages, said passages including first passages for flowing of a first fluid there through and second passages for flowing of a second fluid there through, a plurality of said first passages being serially flow connected to define a first flow path for the first fluid through the heat exchanger, a plurality of said second passages being serially flow connected to define a second flow path for the second fluid through the heat exchanger, each of said flow paths is arranged generally helically characterized in that said first and second flow paths (30, 32) are arranged in an at least partially overlapping relationship to each other, the flow paths being defined by parallel or radially orientated stacks of thin walled circuits of rectangular, triangular or other polygonal cross-section and wherein each flow path (30, 32) is through one of said passages in a first stack of passages, then downwardly and through one of said passages in a second stack of passages adjacent to said first stack, then downwardly and through one of said passages in said first stack, then downwardly and through one of said passages in said second stack, and continuing in this manner until passage through a lowest of said passages in one of said first and second stacks to an outlet from the heat exchanger, wherein the flow for each flow path (32, 30) is through alternate ones of said passages in each pair of said first and second stacks (38,40). two termini (106, 108) of the heat exchanger and is defined by at least three of said walls (44, 46, 48) connected together, wherein each of said walls (44, 46, 48) of one of said passages is also one of said walls (44, 46, 48) of an adjacent one of said passages, said passages including first passages for flowing of a first fluid there through and second passages for flowing of a second fluid there through, a plurality of said first passages being serially flow connected to define a first flow path for the first fluid through the heat exchanger, a plurality of said second passages being serially flow connected to define a second flow path for the second fluid through the heat exchanger, a first flow path for the first fluid through the heat exchanger, a plurality of said second passages being serially flow connected to define a second flow path for the second fluid through the heat exchanger, each of said flow paths is arranged generally helically characterized in that said first and second flow paths (30, 32) are arranged in an at least partially overlapping relationship to each other, the flow paths being defined by parallel or radially orientated stacks of thin walled circuits of rectangular, triangular or other polygonal cross-section and wherein each flow path (30, 32) is through one of said passages in a first stack of passages, then downwardly and through one of said passages in a second stack of passages adjacent to said first stack, then downwardly and through one of said passages in said first stack, then downwardly and through one of said passages in said second stack, and continuing in this manner until passage through a lowest of said passages in one of said first and second stacks to an outlet from the heat exchanger, wherein the flow for each flow path (32, 30) is through alternate ones of said passages in each pair of said first and second stacks (38,40).