A heat exchanger comprising: a core; a first manifold comprising: a primary fluid channel extending between a fluid port and a first branched region; a plurality of secondary fluid channels fluidly connected to the primary fluid channel at the first branched region; and a first overlap region of the plurality of secondary fluid channels downstream of the first branched region and connected to the core at a first transition region; and a second manifold comprising: a primary fluid channel extending between a fluid port and a first branched region; a plurality of secondary fluid channels fluidly connected to the primary fluid channel at the first branched region; and a first overlap region of the plurality of secondary fluid channels downstream of the first branched region and connected to the core at a first transition region; wherein each of the first and second manifolds has a fractal geometry; wherein each of the plurality of secondary fluid channels of the first and second manifolds is tubular between the first branched region and the first transition region; wherein the plurality of secondary fluid channels (24, 34) of the first and second manifolds are interleaved at the first overlap region such that a first layer of secondary fluid channels of the first manifold forms a first flow layer within the core, a first layer of secondary fluid channels of the second manifold forms a second flow layer within the core, and the first flow layer is adjacent and parallel to the second flow layer; wherein each of the plurality of secondary fluid channels of at least one of the first and second manifolds forms a curved path at the respective first overlap region; wherein a cross-sectional area of each of the plurality of secondary fluid channels of the first and second manifolds changes at the first transition region such that the first transition region defines a perpendicular plane through the plurality of secondary fluid channels at which the cross-sectional area of each of the plurality of secondary fluid channels is hexagonal; and wherein the core is a three-dimensional honeycomb structure. a core; a first manifold comprising: a primary fluid channel extending between a fluid port and a first branched region; a plurality of secondary fluid channels fluidly connected to the primary fluid channel at the first branched region; and a first overlap region of the plurality of secondary fluid channels downstream of the first branched region and connected to the core at a first transition region; and a primary fluid channel extending between a fluid port and a first branched region; a plurality of secondary fluid channels fluidly connected to the primary fluid channel at the first branched region; and a first overlap region of the plurality of secondary fluid channels downstream of the first branched region and connected to the core at a first transition region; and a second manifold comprising: a primary fluid channel extending between a fluid port and a first branched region; a plurality of secondary fluid channels fluidly connected to the primary fluid channel at the first branched region; and a first overlap region of the plurality of secondary fluid channels downstream of the first branched region and connected to the core at a first transition region; a primary fluid channel extending between a fluid port and a first branched region; a plurality of secondary fluid channels fluidly connected to the primary fluid channel at the first branched region; and a first overlap region of the plurality of secondary fluid channels downstream of the first branched region and connected to the core at a first transition region; wherein each of the first and second manifolds has a fractal geometry; wherein each of the plurality of secondary fluid channels of the first and second manifolds is tubular between the first branched region and the first transition region; wherein the plurality of secondary fluid channels (24, 34) of the first and second manifolds are interleaved at the first overlap region such that a first layer of secondary fluid channels of the first manifold forms a first flow layer within the core, a first layer of secondary fluid channels of the second manifold forms a second flow layer within the core, and the first flow layer is adjacent and parallel to the second flow layer; wherein each of the plurality of secondary fluid channels of at least one of the first and second manifolds forms a curved path at the respective first overlap region; wherein a cross-sectional area of each of the plurality of secondary fluid channels of the first and second manifolds changes at the first transition region such that the first transition region defines a perpendicular plane through the plurality of secondary fluid channels at which the cross-sectional area of each of the plurality of secondary fluid channels is hexagonal; and wherein the core is a three-dimensional honeycomb structure.