A heat exchanger comprising two tanks, and a plurality of tubes for a medium which is to be cooled/heated are arranged in stacked, transverse rows, with the tubes arrayed to extend from the entrance to the exit side of the heat exchanger. The tubes are disposed between and transmit the medium between the tanks. Ducts between the stacked rows of the tubes for flow of a different cooling/heating medium. The ducts are arranged to run transversely to the longitudinal direction of the tubes. The opposing connection surfaces of the tanks converge in the direction of flow of the cooling/heating medium through the heat exchanger. The ducts closer to the connection surfaces are angled convergingly like the connection surfaces. Such a heat exchanger may be used, e.g. as a cooler, and in a motor vehicle.