A heat exchanger includes a core section defining a plurality of first fluid channels and a plurality of second fluid channels. The heat exchanger also includes a header section defining a plurality of first fluid layers and a plurality of second fluid layers. The heat exchanger further includes a transition region located between the header section and the core section, the transition region fluidly coupling the plurality of first fluid layers to the first fluid channels, each of the first fluid layers routing a first fluid to a respective group of first fluid channels.