A heat exchanger (4) has fluid flow channels (6) with at least one heat exchanging surface (10) which has an undulating surface section for which the surface profile varies along a predetermined direction such that at a first edge (E1) the surface profile follows a first transverse wave (20), at a second edge (E)2 the surface profile follows a second transverse wave (22) and at an intermediate point I between the edges the surface profile follows a third transverse wave (24). The third transverse wave (24) has a different phase, frequency or amplitude to the first and second transverse waves so that chevron-shaped ridges and valleys are formed. This improves the mixing of fluid passing through the channel and hence the heat exchange efficiency.