A heat exchanger comprising: a plurality of fluid flow channels; at least one of the fluid flow channels comprising at least one heat exchanging surface comprising at least one undulating surface section extending along at least part of a length of the channel, wherein the at least one heat exchanging surface comprises a secondary surface of the heat exchanger; wherein for each undulating surface section: along a first edge of the undulating surface section aligned with a predetermined direction, a profile of the heat exchanging surface varies according to a first transverse wave with a direction of travel corresponding to the predetermined direction; along a second edge of the undulating surface section aligned with the predetermined direction, a profile of the heat exchanging surface varies according to a second transverse wave with a direction of travel corresponding to the predetermined direction, characterised in that: at an intermediate portion of the undulating surface section lying between the first edge and the second edge, a profile of the heat exchanging surface varies according to a third transverse wave with a direction of travel corresponding to the predetermined direction; wherein said third transverse wave is at least out of phase with at least one of said first transverse wave and said second transverse wave, to provide one or more chevron-shaped ridges or valleys extending across a width of the undulating surface section and pointing along the predetermined direction such that an apex of the chevron-shaped ridges or valleys occurs at a different position along the predetermined direction at the intermediate portion of the undulating surface section than at the first and second edges. a plurality of fluid flow channels; at least one of the fluid flow channels comprising at least one heat exchanging surface comprising at least one undulating surface section extending along at least part of a length of the channel, wherein the at least one heat exchanging surface comprises a secondary surface of the heat exchanger; wherein for each undulating surface section: along a first edge of the undulating surface section aligned with a predetermined direction, a profile of the heat exchanging surface varies according to a first transverse wave with a direction of travel corresponding to the predetermined direction; along a second edge of the undulating surface section aligned with the predetermined direction, a profile of the heat exchanging surface varies according to a second transverse wave with a direction of travel corresponding to the predetermined direction, characterised in that: at an intermediate portion of the undulating surface section lying between the first edge and the second edge, a profile of the heat exchanging surface varies according to a third transverse wave with a direction of travel corresponding to the predetermined direction; wherein said third transverse wave is at least out of phase with at least one of said first transverse wave and said second transverse wave, to provide one or more chevron-shaped ridges or valleys extending across a width of the undulating surface section and pointing along the predetermined direction such that an apex of the chevron-shaped ridges or valleys occurs at a different position along the predetermined direction at the intermediate portion of the undulating surface section than at the first and second edges. along a first edge of the undulating surface section aligned with a predetermined direction, a profile of the heat exchanging surface varies according to a first transverse wave with a direction of travel corresponding to the predetermined direction; along a second edge of the undulating surface section aligned with the predetermined direction, a profile of the heat exchanging surface varies according to a second transverse wave with a direction of travel corresponding to the predetermined direction, characterised in that: at an intermediate portion of the undulating surface section lying between the first edge and the second edge, a profile of the heat exchanging surface varies according to a third transverse wave with a direction of travel corresponding to the predetermined direction; wherein said third transverse wave is at least out of phase with at least one of said first transverse wave and said second transverse wave, to provide one or more chevron-shaped ridges or valleys extending across a width of the undulating surface section and pointing along the predetermined direction such that an apex of the chevron-shaped ridges or valleys occurs at a different position along the predetermined direction at the intermediate portion of the undulating surface section than at the first and second edges. at an intermediate portion of the undulating surface section lying between the first edge and the second edge, a profile of the heat exchanging surface varies according to a third transverse wave with a direction of travel corresponding to the predetermined direction; wherein said third transverse wave is at least out of phase with at least one of said first transverse wave and said second transverse wave, to provide one or more chevron-shaped ridges or valleys extending across a width of the undulating surface section and pointing along the predetermined direction such that an apex of the chevron-shaped ridges or valleys occurs at a different position along the predetermined direction at the intermediate portion of the undulating surface section than at the first and second edges.