Abstract: Heat exchanger, in particular for cooling power electronics, comprising an insulating element which separates a first fluid medium, which is in contact with a heat source, from a second fluid medium, which differs in at least one property from the first fluid medium and which is in fluid connection with a heat sink or is itself a heat sink, and a heat transfer element which has a higher thermal conductivity than the insulating element, wherein the insulating element comprises at least a first passage opening in which a first heat transfer element is arranged, wherein the heat transfer element is thermally connected both to the first fluid medium and/or the heat source, and to the second fluid medium and is fluidically sealed with respect to the insulating element by means of a sealing element.