The invention relates to a heat exchanger (1), especially for motor vehicles, which comprises flat pipes (2) through whose interior a first fluid (FL1) flows and that can be impinged upon externally by a second fluid (FL2). The flat pipes (2) are substantially disposed at an angle to the direction of flow (S2) of the second fluid (FL2) and parallel relative one another and are spaced apart so as to configure flow paths for the second fluid (FL2) that extend through the heat exchanger. Cooling ribs (3) are disposed in the flow paths and extend between respective adjacent flat pipes (2). A plurality of wavy ribs (3) are provided as the cooling ribs. These wavy ribs are disposed one behind the other in the direction of flow (S2) of the second fluid (FL2) and are off-set from one another in the direction of flow (S1) of the first fluid (FL1).