A two-start helical heat exchanger comprises a helical baffle extending along a length of the heat exchanger and having first and second surfaces, wherein: the first surface of the baffle is arranged to provide a first helical fluid flow path for a first fluid; and the second surface of the baffle is arranged to provide a second helical fluid flow path for a second fluid, wherein the second fluid flow path is arranged in counter-flow with the first fluid flow path and a casing within which the baffle is mounted. The baffle is arranged such that first and second fluid flow paths are in thermal contact with each other through the baffle, and in thermal contact with the casing. The heat exchanger may be incorporated into a power converter, for example to cool the power converter. The heat exchanger may be used on an aircraft.