A heat exchanger (4) comprises a heat exchanger core (20) comprising first fluid channels (22) and second fluid flow channels (24) for exchange of heat between the first and second fluids. First and second manifold portions (42, 44) are provided to guide the first and second fluids between the first and second fluid flow channels (22, 249 and first and second fluid interface portions (48, 49) which comprise fewer channels than the heat exchanger core (20). The first manifold portion (42) includes at least one tunnel portion (46) extending through the second manifold portion (44) at an angle to the direction of second fluid flow. Hence at least part of the first fluid is directed through the inside of the tunnel portion while the second fluid passes around the outside of the tunnel portion. This enables more compact heat exchanger design.