A heat exchanger and a method for additively manufacturing the heat exchanger are provided. The heat exchanger includes a plurality of fluid passageways that are formed by additive manufacturing methods which enable the formation of fluid passageways that are smaller in size, that have thinner walls, and that have complex and intricate heat exchanger features that were not possible using prior manufacturing methods. For example, the fluid passageways may be curvilinear and may include heat exchanging fins that are less than 0.01 inches thick and formed at a fin density of more than four heat exchanging fins per centimeter. In addition, the heat exchanging fins may be angled with respect to the walls of the fluid passageways and adjacent fins may be offset relative to each other.