The invention relates to a heat exchanger with at least one duct, which can be flowed through by flowing medium from an inlet cross-section to an outlet cross-section, has an inside and outside, and which comprises, on the inside, structural elements for increasing the transfer of heat. The invention provides that the structural elements (11) are variably arranged and/or configured in the direction of flow (P) so that the duct (10), on the inside, has a variable heat transfer that, in particular, increases in the direction of flow (P).