A heat exchanger includes a core comprising a single piece continuous boundary having a first surface defining a first labyrinth, and an opposing second surface defining a second labyrinth; a first inlet manifold connected to the first labyrinth and configured to supply a first fluid to the first labyrinth; and a second inlet manifold connected to the second labyrinth and configured to supply a second fluid to the second labyrinth; wherein the core comprises a plurality of identical three dimensional unit cell structures replicated in three orthogonal spatial dimensions.