The invention relates to a heat exchanger of a plate-type construction having first flow channels for a first medium, each formed by a pair of plates, and having corrugated ribs, around which a second medium flows, arranged between neighboring plate pairs. In order to provide an improved heat exchanger which allows a more compact construction of the plant when used in a heating or air conditioning unit of a motor vehicle, it is proposed that the corrugated ribs and at least one of the neighboring plate pairs form second flow channels for the second medium and the second flow channels have a curved course, so that the second medium undergoes a change of flow direction on flowing through the heat exchanger.