An electric machine comprises a rotor, preferably having permanent magnets, although other field generation means are available, and a stator. The stator has coils wound on stator bars for interaction with the magnetic field of the rotor across an air gap defined between them. The rotor comprises a housing of a chamber containing refrigerant. The rotor housing has heat dissipating fins accessible by the open environment whereby air movement relative to the housing caused at least by rotation of the rotor absorbs heat from the fins. The machine may be an axial flux machine, the coils being wound on bars that are disposed circumferentially spaced around a fixed axle of the machine forming a rotational axis of the rotor. The machine may be a wheel motor for a vehicle, wherein the wheel is mounted directly on the rotor housing.