A heat exchanger includes a heat exchanger core, a header defining a header manifold, and a transition portion that provides fluid communication between the heat exchanger core and the header manifold. The transition portion includes a transition tube extending between the header and the heat exchanger core, a header junction where the transition tube joins the header, and a splitting junction that splits the transition tube into the plurality of heat exchange tubes. The header junction may define elliptical inlet apertures, a large filleted joint, and a junction thickness that is greater than a header wall thickness.